Move `public/*` to `public/assets/*`

Some old PRs (like #15219) introduced inconsistent directory system.

For example: why the local directory "public" is accessed by
`http://site/assets`? How to serve the ".well-known" files properly in
the public directory?

For convention rules, the "public" directory is widely used for the
website's root directory. It shouldn't be an exception for Gitea.

So, this PR makes the things consistent:

* `http://site/assets/foo` means `{CustomPath}/public/assets/foo`.
* `{CustomPath}/public/.well-known` and `{CustomPath}/public/robots.txt`
can be used in the future.

This PR is also a prerequisite for a clear solution for:
* #21942
* #25892 
* discourse.gitea.io: [.well-known path serving custom files behind
proxy?](https://discourse.gitea.io/t/well-known-path-serving-custom-files-behind-proxy/5445/1)

This PR is breaking for users who have custom "public" files (CSS/JS).
After getting approvals, I will update the documents.

----

## ⚠️ BREAKING ⚠️

If you have files in your "custom/public/" folder, please move them to
"custom/public/assets/".

---------

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: Giteabot <teabot@gitea.io>

* Support unicode emojis and remove emojify.js

This PR replaces all use of emojify.js and adds unicode emoji support to various areas of gitea.

This works in a few ways:

First it adds emoji parsing support into gitea itself. This allows us to

 * Render emojis from valid alias (:smile:)
 * Detect unicode emojis and let us put them in their own class with proper aria-labels and styling
 * Easily allow for custom "emoji"
 * Support all emoji rendering and features without javascript
 * Uses plain unicode and lets the system render in appropriate emoji font
 * Doesn't leave us relying on external sources for updates/fixes/features

That same list of emoji is also used to create a json file which replaces the part of emojify.js that populates the emoji search tribute. This file is about 35KB with GZIP turned on and I've set it to load after the page renders to not hinder page load time (and this removes loading emojify.js also)

For custom "emoji" it uses a pretty simple scheme of just looking for /emojis/img/name.png where name is something a user has put in the "allowed reactions" setting we already have. The gitea reaction that was previously hard coded into a forked copy of emojify.js is included and works as a custom reaction under this method.

The emoji data sourced here is from https://github.com/github/gemoji which is the gem library Github uses for their emoji rendering (and a data source for other sites). So we should be able to easily render any emoji and :alias: that Github can, removing any errors from migrated content. They also update it as well, so we can sync when there are new unicode emoji lists released.

I've included a slimmed down and slightly modified forked copy of https://github.com/knq/emoji to make up our own emoji module. The code is pretty straight forward and again allows us to have a lot of flexibility in what happens.

I had seen a few comments about performance in some of the other threads if we render this ourselves, but there doesn't seem to be any issue here. In a test it can parse, convert, and render 1,000 emojis inside of a large markdown table in about 100ms on my laptop (which is many more emojis than will ever be in any normal issue). This also prevents any flickering and other weirdness from using javascript to render some things while using go for others.

Not included here are image fall back URLS. I don't really think they are necessary for anything new being written in 2020. However, managing the emoji ourselves would allow us to add these as a feature later on if it seems necessary.

Fixes: https://github.com/go-gitea/gitea/issues/9182
Fixes: https://github.com/go-gitea/gitea/issues/8974
Fixes: https://github.com/go-gitea/gitea/issues/8953
Fixes: https://github.com/go-gitea/gitea/issues/6628
Fixes: https://github.com/go-gitea/gitea/issues/5130

* add new shared function emojiHTML

* don't increase emoji size in issue title

* Update templates/repo/issue/view_content/add_reaction.tmpl

Co-Authored-By: 6543 <6543@obermui.de>

* Support for emoji rendering in various templates

* Render code and review comments as they should be

* Better way to handle mail subjects

* insert unicode from tribute selection

* Add template helper for plain text when needed

* Use existing replace function I forgot about

* Don't include emoji greater than Unicode Version 12

Only include emoji and aliases in JSON

* Update build/generate-emoji.go

* Tweak regex slightly to really match everything including random invisible characters. Run tests for every emoji we have

* final updates

* code review

* code review

* hard code gitea custom emoji to match previous behavior

* Update .eslintrc

Co-Authored-By: silverwind <me@silverwind.io>

* disable preempt

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: Lauris BH <lauris@nix.lv>
Co-authored-by: guillep2k <18600385+guillep2k@users.noreply.github.com>