[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

modules/setting/setting.go

@@ -169,6 +169,7 @@ var (
 		DefaultShowFullName   bool
 		DefaultTheme          string
 		Themes                []string
+		Reactions             []string
 		SearchRepoDescription bool
 		UseServiceWorker      bool
 
@@ -198,6 +199,7 @@ var (
 		MaxDisplayFileSize:  8388608,
 		DefaultTheme:        `gitea`,
 		Themes:              []string{`gitea`, `arc-green`},
+		Reactions:           []string{`+1`, `-1`, `laugh`, `hooray`, `confused`, `heart`, `rocket`, `eyes`},
 		Admin: struct {
 			UserPagingNum   int
 			RepoPagingNum   int