- Added [GitHub's `relative-time` element](https://github.com/github/relative-time-element)
- Converted all formatted timestamps to use this element
- No more flashes of unstyled content around time elements
- These elements are localized using the `lang` property of the HTML file
- Relative (e.g. the activities in the dashboard) and duration (e.g.
server uptime in the admin page) time elements are auto-updated to keep
up with the current time without refreshing the page
- Code that is not needed anymore such as `formatting.js` and parts of `since.go` have been deleted

Replaces #21440
Follows #22861

## Screenshots

### Localized

![image](https://user-images.githubusercontent.com/20454870/230775041-f0af4fda-8f6b-46d3-b8e3-d340c791a50c.png)

![image](https://user-images.githubusercontent.com/20454870/230673393-931415a9-5729-4ac3-9a89-c0fb5fbeeeb7.png)

### Tooltips

#### Native for dates

![image](https://user-images.githubusercontent.com/20454870/230797525-1fa0a854-83e3-484c-9da5-9425ab6528a3.png)

#### Interactive for relative

![image](https://user-images.githubusercontent.com/115237/230796860-51e1d640-c820-4a34-ba2e-39087020626a.png)

### Auto-update

![rec](https://user-images.githubusercontent.com/20454870/230672159-37480d8f-435a-43e9-a2b0-44073351c805.gif)

---------

Signed-off-by: Yarden Shoham <git@yardenshoham.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: delvh <dev.lh@web.de>

- Currently, the 'IsZero' function for 'TimeStamp' just checks if the
unix time is zero, which is not the behavior of 'Time.IsZero()', but
Gitea is using this method in accordance with the behavior of
'Time.IsZero()'.
- Adds a new condition to check for the zero time instant.
- Fixes a bug where non-expiring GPG keys where shown as they expired on
Jan 01, 0001.
- Related https://codeberg.org/Codeberg/Community/issues/791

Before:

![image](https://user-images.githubusercontent.com/25481501/208509035-ecc5fa4a-3bd1-4fa3-beba-90875719163c.png)

After:

![image](https://user-images.githubusercontent.com/25481501/208508950-3e7f6eeb-be83-432a-89a6-d738553dafe4.png)

Change all license headers to comply with REUSE specification.

Fix #16132

Co-authored-by: flynnnnnnnnnn <flynnnnnnnnnn@github>
Co-authored-by: John Olheiser <john.olheiser@gmail.com>

Our implementation of `IsZero` can't work. An "empty" timestamp (= 0)
calls `time.Unix(int64(ts), 0).IsZero()` which is always `false`. Only
`time.Time{}.IsZero()` is `true`.
We call this method ~~only at one place~~ and there the value
(`UpdatedUnix`) should be always != 0 so this PR may not have
consequences.

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

* Replace ServeStream with ServeContent.

* Update modules/timeutil/timestamp.go

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

Co-authored-by: delvh <dev.lh@web.de>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>

Add nakedret, unconvert, wastedassign, stylecheck and nolintlint linters to improve code readability

- nakedret - https://github.com/alexkohler/nakedret - nakedret is a Go static analysis tool to find naked returns in functions greater than a specified function length.
- unconvert - https://github.com/mdempsky/unconvert - Remove unnecessary type conversions
- wastedassign - https://github.com/sanposhiho/wastedassign -  wastedassign finds wasted assignment statements.
- notlintlint -  Reports ill-formed or insufficient nolint directives
- stylecheck - https://staticcheck.io/docs/checks/#ST - keep style consistent
  - excluded: [ST1003 - Poorly chosen identifier](https://staticcheck.io/docs/checks/#ST1003) and [ST1005 - Incorrectly formatted error string](https://staticcheck.io/docs/checks/#ST1005)

Signed-off-by: jolheiser <john.olheiser@gmail.com>

* Fix timezone on issue deadline

* FormatDate

Co-authored-by: zeripath <art27@cantab.net>

* display ui time with customize time location

* fix lint

* rename UILocation to DefaultUILocation

* move time related functions to modules/timeutil

* fix tests

* fix tests

* fix build

* fix swagger

* improvements for supporting UI Location

* improved the comment

* refactor struct's time to remove unnecessary memory usage

* use AsTimePtr simple code

* fix tests

* fix time compare

* fix template on gpg

* use AddDuration instead of Add