[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.tmpl

@@ -28,7 +28,7 @@
 					{{end}}
 						{{if not $.Repository.IsArchived}}
 							<div class="ui right actions">
-								{{template "repo/issue/view_content/add_reaction" Dict "ctx" $ "ActionURL" (Printf "%s/issues/%d/reactions" $.RepoLink .Issue.Index) }}
+								{{template "repo/issue/view_content/add_reaction" Dict "ctx" $ "ActionURL" (Printf "%s/issues/%d/reactions" $.RepoLink .Issue.Index) "AllowedReactions" $.AllowedReactions}}
 								{{template "repo/issue/view_content/context_menu" Dict "ctx" $ "item" .Issue "delete" false "diff" false }}
 							</div>
 						{{end}}
@@ -47,7 +47,7 @@
 					{{$reactions := .Issue.Reactions.GroupByType}}
 					{{if $reactions}}
 						<div class="ui attached segment reactions">
-							{{template "repo/issue/view_content/reactions" Dict "ctx" $ "ActionURL" (Printf "%s/issues/%d/reactions" $.RepoLink .Issue.Index) "Reactions" $reactions }}
+							{{template "repo/issue/view_content/reactions" Dict "ctx" $ "ActionURL" (Printf "%s/issues/%d/reactions" $.RepoLink .Issue.Index) "Reactions" $reactions "AllowedReactions" $.AllowedReactions}}
 						</div>
 					{{end}}
 					{{if .Issue.Attachments}}