[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

templates/repo/diff/comments.tmpl

@@ -38,7 +38,7 @@
 		{{$reactions := .Reactions.GroupByType}}
 		{{if $reactions}}
 			<div class="ui attached segment reactions">
-			{{template "repo/issue/view_content/reactions" Dict "ctx" $ "ActionURL" (Printf "%s/comments/%d/reactions" $.root.RepoLink .ID) "Reactions" $reactions }}
+			{{template "repo/issue/view_content/reactions" Dict "ctx" $ "ActionURL" (Printf "%s/comments/%d/reactions" $.root.RepoLink .ID) "Reactions" $reactions "AllowedReactions" $.AllowedReactions }}
 			</div>
 		{{end}}
 	</div>