[Feature] Custom Reactions (#8886)

* add [ui] Reactions

* move contend check from form to go functions

* use else if

* check if reaction is allowed only on react
(so previous custom reaction can be still removed)

* use $.AllowedReactions in templates

* use ctx.Flash.Error

* use it there too

* add redirection

* back to server error
because a wrong reaction is a template issue ...

* add emoji list link

* add docs entry

* small wording nit
suggestions from @jolheiser - thx

* same reactions as github

* fix PR reactions

* handle error so template JS could check

* Add Integrations Test

* add REACTIONS setting to cheat-sheet doc page

docs/content/doc/advanced/config-cheat-sheet.en-us.md

@@ -118,6 +118,7 @@ Values containing `#` or `;` must be quoted using `` ` `` or `"""`.
 - `DEFAULT_THEME`: **gitea**: \[gitea, arc-green\]: Set the default theme for the Gitea install.
 - `THEMES`:  **gitea,arc-green**: All available themes. Allow users select personalized themes
   regardless of the value of `DEFAULT_THEME`.
+- `REACTIONS`: All available reactions. Allow users react with different emoji's.
 - `DEFAULT_SHOW_FULL_NAME`: **false**: Whether the full name of the users should be shown where possible. If the full name isn't set, the username will be used.
 - `SEARCH_REPO_DESCRIPTION`: **true**: Whether to search within description at repository search on explore page.
 - `USE_SERVICE_WORKER`: **true**: Whether to enable a Service Worker to cache frontend assets.