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

model/config.go

@@ -226,6 +226,7 @@ type TeamSettings struct {
 	RestrictPrivateChannelManagement *string
 	UserStatusAwayTimeout            *int64
 	MaxChannelsPerTeam               *int64
+	MaxNotificationsPerChannel       *int64
 }
 
 type LdapSettings struct {
@@ -507,6 +508,11 @@ func (o *Config) SetDefaults() {
 		*o.TeamSettings.MaxChannelsPerTeam = 2000
 	}
 
+	if o.TeamSettings.MaxNotificationsPerChannel == nil {
+		o.TeamSettings.MaxNotificationsPerChannel = new(int64)
+		*o.TeamSettings.MaxNotificationsPerChannel = 1000
+	}
+
 	if o.EmailSettings.EnableSignInWithEmail == nil {
 		o.EmailSettings.EnableSignInWithEmail = new(bool)
 
@@ -1003,6 +1009,10 @@ func (o *Config) IsValid() *AppError {
 		return NewLocAppError("Config.IsValid", "model.config.is_valid.max_channels.app_error", nil, "")
 	}
 
+	if *o.TeamSettings.MaxNotificationsPerChannel <= 0 {
+		return NewLocAppError("Config.IsValid", "model.config.is_valid.max_notify_per_channel.app_error", nil, "")
+	}
+
 	if !(*o.TeamSettings.RestrictDirectMessage == DIRECT_MESSAGE_ANY || *o.TeamSettings.RestrictDirectMessage == DIRECT_MESSAGE_TEAM) {
 		return NewLocAppError("Config.IsValid", "model.config.is_valid.restrict_direct_message.app_error", nil, "")
 	}