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

i18n/en.json

@@ -675,6 +675,18 @@
     "id": "api.context.invalid_param.app_error",
     "translation": "Invalid {{.Name}} parameter"
   },
+  {
+    "id": "api.post.disabled_here",
+    "translation": "@here has been disabled because the channel has more than {{.Users}} users."
+  },
+  {
+    "id": "api.post.disabled_all",
+    "translation": "@all has been disabled because the channel has more than {{.Users}} users."
+  },
+  {
+    "id": "api.post.disabled_channel",
+    "translation": "@channel has been disabled because the channel has more than {{.Users}} users."
+  },
   {
     "id": "api.context.invalid_team_url.debug",
     "translation": "TeamURL accessed when not valid. Team URL should not be used in api functions or those that are team independent"
@@ -3331,6 +3343,10 @@
     "id": "model.config.is_valid.max_channels.app_error",
     "translation": "Invalid maximum channels per team for team settings.  Must be a positive number."
   },
+  {
+    "id": "model.config.is_valid.max_notify_per_channel.app_error",
+    "translation": "Invalid maximum notifications per channel for team settings.  Must be a positive number."
+  },
   {
     "id": "model.config.is_valid.max_file_size.app_error",
     "translation": "Invalid max file size for file settings. Must be a zero or positive number."