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/web_hub.go

@@ -102,6 +102,11 @@ func PublishSkipClusterSend(message *model.WebSocketEvent) {
 	}
 }
 
+func InvalidateCacheForChannel(channelId string) {
+	Srv.Store.User().InvalidateProfilesInChannelCache(channelId)
+	Srv.Store.Channel().InvalidateMemberCount(channelId)
+}
+
 func InvalidateCacheForUser(userId string) {
 	InvalidateCacheForUserSkipClusterSend(userId)