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

config/config.json

@@ -51,7 +51,8 @@
         "RestrictPublicChannelManagement": "all",
         "RestrictPrivateChannelManagement": "all",
         "UserStatusAwayTimeout": 300,
-        "MaxChannelsPerTeam": 2000
+        "MaxChannelsPerTeam": 2000,
+        "MaxNotificationsPerChannel": 1000
     },
     "SqlSettings": {
         "DriverName": "mysql",