* 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
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