[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/customizing-gitea.en-us.md

@@ -161,6 +161,15 @@ Locales may change between versions, so keeping track of your customized locales
 
 To add a custom Readme, add a markdown formatted file (without an `.md` extension) to `custom/options/readme`
 
+### Reactions
+
+To change reaction emoji's you can set allowed reactions at app.ini
+```
+[ui]
+REACTIONS = +1, -1, laugh, confused, heart, hooray, eyes
+```
+A full list of supported emoji's is at [emoji list](https://gitea.com/gitea/gitea.com/issues/8)
+
 ## Customizing the look of Gitea
 
 As of version 1.6.0 Gitea has built-in themes. The two built-in themes are, the default theme `gitea`, and a dark theme `arc-green`. To change the look of your Gitea install change the value of `DEFAULT_THEME` in the [ui](https://docs.gitea.io/en-us/config-cheat-sheet/#ui-ui) section of `app.ini` to another one of the available options.