PLT-4429 disabling at_all at_channel metions mentions when channel has more than 1k users (#4627)

* PLT-4429 disabling explicit mentions when channel has more than 1k users

* Fixing test case

* Adding setting to the admin console

* Fixing bad translation

api/team.go

@@ -339,7 +339,7 @@ func LeaveTeam(team *model.Team, user *model.User) *model.AppError {
 
 	for _, channel := range *channelList {
 		if channel.Type != model.CHANNEL_DIRECT {
-			Srv.Store.User().InvalidateProfilesInChannelCache(channel.Id)
+			InvalidateCacheForChannel(channel.Id)
 			if result := <-Srv.Store.Channel().RemoveMember(channel.Id, user.Id); result.Err != nil {
 				return result.Err
 			}