We have to define this one in helpers.css because tailwind only
generates a single class but certain things rely on this being
double-class. Command ran:

```sh
perl -p -i -e 's#gt-hidden#tw-hidden#g' web_src/js/**/* templates/**/* models/**/* web_src/css/**/*

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

Migrate `gt-font-*` to `tw-text-*` All tailwind-original class names are
also available and render like they would with 16px root font size.

We currently have root font size at 14px, but I would like to eventually
migrate us to 16px so that the tailwind docs apply to us unchangend and
because 16px is the recommended root font size for web pages in general.
Also the number 16 is much better dividable than 14 so will result in
more integers.

Commands ran:

```sh
perl -p -i -e 's#gt-font-light#tw-font-light#g' web_src/js/**/* templates/**/*
perl -p -i -e 's#gt-font-normal#tw-font-normal#g' web_src/js/**/* templates/**/*
perl -p -i -e 's#gt-font-medium#tw-font-medium#g' web_src/js/**/* templates/**/*
perl -p -i -e 's#gt-font-semibold#tw-font-semibold#g' web_src/js/**/* templates/**/*
perl -p -i -e 's#gt-font-bold#tw-font-bold#g' web_src/js/**/* templates/**/*
```

Used all existing css vars, other migrations are 1:1.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

We need to scan `.go` files for tailwind classes. Does not seem to
affect build time much luckily.

Fixes:
https://github.com/go-gitea/gitea/pull/29678#discussion_r1518448600

Verified via `rg tw-object-contain public/assets/css/index.css`.

---------

Co-authored-by: Giteabot <teabot@gitea.io>

Tailwind content is not going to appear in `web_src/css`,
`web_src/fomantic` or `web_src/svg` or the JSON templates, so we don't
need to have tailwind scan these directories which will speed up the
build.

---------

Co-authored-by: delvh <dev.lh@web.de>

Get rid of the `postcss@7` dependency by inlining this simple function.

As per discussion in https://github.com/go-gitea/gitea/pull/29423, I
think this is the right way that does not burden developers having to
think about CSS precedence which should be irrelevant with an atomic CSS
framework.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

This will get tailwindcss working on a basic level. It provides only the
utility classes, e.g. no tailwind base which we don't need because we
have our own CSS reset. Without the base, we also do not have their CSS
variables so a small amount of features do not work and I removed the
generated classes for them.

***Note for future developers: This currently uses a `tw-` prefix, so we
use it like `tw-p-3`.***

<details>
<summary>Currently added CSS, all false-positives</summary>

```
.\!visible{

    visibility: visible !important
}

.visible{

    visibility: visible
}

.invisible{

    visibility: hidden
}

.collapse{

    visibility: collapse
}

.static{

    position: static
}

.\!fixed{

    position: fixed !important
}

.absolute{

    position: absolute
}

.relative{

    position: relative
}

.sticky{

    position: sticky
}

.left-10{

    left: 2.5rem
}

.isolate{

    isolation: isolate
}

.float-right{

    float: right
}

.float-left{

    float: left
}

.mr-2{

    margin-right: 0.5rem
}

.mr-3{

    margin-right: 0.75rem
}

.\!block{

    display: block !important
}

.block{

    display: block
}

.inline-block{

    display: inline-block
}

.inline{

    display: inline
}

.flex{

    display: flex
}

.inline-flex{

    display: inline-flex
}

.\!table{

    display: table !important
}

.inline-table{

    display: inline-table
}

.table-caption{

    display: table-caption
}

.table-cell{

    display: table-cell
}

.table-column{

    display: table-column
}

.table-column-group{

    display: table-column-group
}

.table-footer-group{

    display: table-footer-group
}

.table-header-group{

    display: table-header-group
}

.table-row-group{

    display: table-row-group
}

.table-row{

    display: table-row
}

.flow-root{

    display: flow-root
}

.inline-grid{

    display: inline-grid
}

.contents{

    display: contents
}

.list-item{

    display: list-item
}

.\!hidden{

    display: none !important
}

.hidden{

    display: none
}

.flex-shrink{

    flex-shrink: 1
}

.shrink{

    flex-shrink: 1
}

.flex-grow{

    flex-grow: 1
}

.grow{

    flex-grow: 1
}

.border-collapse{

    border-collapse: collapse
}

.select-all{

    user-select: all
}

.resize{

    resize: both
}

.flex-wrap{

    flex-wrap: wrap
}

.overflow-visible{

    overflow: visible
}

.rounded{

    border-radius: 0.25rem
}

.border{

    border-width: 1px
}

.text-justify{

    text-align: justify
}

.uppercase{

    text-transform: uppercase
}

.lowercase{

    text-transform: lowercase
}

.capitalize{

    text-transform: capitalize
}

.italic{

    font-style: italic
}

.text-red{

    color: var(--color-red)
}

.text-shadow{

    color: var(--color-shadow)
}

.underline{

    text-decoration-line: underline
}

.overline{

    text-decoration-line: overline
}

.line-through{

    text-decoration-line: line-through
}

.outline{

    outline-style: solid
}

.ease-in{

    transition-timing-function: cubic-bezier(0.4, 0, 1, 1)
}

.ease-in-out{

    transition-timing-function: cubic-bezier(0.4, 0, 0.2, 1)
}

.ease-out{

    transition-timing-function: cubic-bezier(0, 0, 0.2, 1)
}
```

</details>

---------

Co-authored-by: Giteabot <teabot@gitea.io>