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

webapp/utils/constants.jsx

@@ -821,6 +821,7 @@ export const Constants = {
     DEFAULT_MAX_USERS_PER_TEAM: 50,
     MIN_TEAMNAME_LENGTH: 2,
     DEFAULT_MAX_CHANNELS_PER_TEAM: 2000,
+    DEFAULT_MAX_NOTIFICATIONS_PER_CHANNEL: 1000,
     MAX_TEAMNAME_LENGTH: 15,
     MIN_USERNAME_LENGTH: 3,
     MAX_USERNAME_LENGTH: 22,