[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/issue/view_content/reactions.tmpl

@@ -2,14 +2,14 @@
 	<a class="ui label basic{{if $value.HasUser $.ctx.SignedUserID}} blue{{end}}{{if not $.ctx.IsSigned}} disabled{{end}} has-emoji" data-title="{{$value.GetFirstUsers}}{{if gt ($value.GetMoreUserCount) 0}} {{ $.ctx.i18n.Tr "repo.reactions_more" $value.GetMoreUserCount}}{{end}}" data-content="{{ $key }}" data-action-url="{{ $.ActionURL }}">
 		{{if eq $key "hooray"}}
 			:tada:
+		{{else if eq $key "laugh"}}
+			:laughing:
 		{{else}}
-			{{if eq $key "laugh"}}
-				:laughing:
-			{{else}}
-				:{{$key}}:
-			{{end}}
+			:{{$key}}:
 		{{end}}
 		{{len $value}}
 	</a>
 {{end}}
-{{template "repo/issue/view_content/add_reaction" Dict "ctx" $.ctx "ActionURL" .ActionURL }}
+{{if $.AllowedReactions}}
+	{{template "repo/issue/view_content/add_reaction" Dict "ctx" $.ctx "ActionURL" .ActionURL "AllowedReactions" $.AllowedReactions}}
+{{end}}