MM-26206: Add GroupMentions permissions in default channel admin (#14838)

* MM-26206: Add GroupMentions permissions in default channel admin

This permission was skipped from the default permissions function.
Which led to a bug where permissions reset would not bring back
this permission to the different roles.

* Added tests

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

app/app_test.go

@@ -111,6 +111,7 @@ func TestDoAdvancedPermissionsMigration(t *testing.T) {
 		},
 		"channel_admin": {
 			model.PERMISSION_MANAGE_CHANNEL_ROLES.Id,
+			model.PERMISSION_USE_GROUP_MENTIONS.Id,
 		},
 		"team_user": {
 			model.PERMISSION_LIST_TEAM_CHANNELS.Id,
@@ -229,6 +230,7 @@ func TestDoAdvancedPermissionsMigration(t *testing.T) {
 			model.PERMISSION_MANAGE_OTHERS_SLASH_COMMANDS.Id,
 			model.PERMISSION_MANAGE_INCOMING_WEBHOOKS.Id,
 			model.PERMISSION_MANAGE_OUTGOING_WEBHOOKS.Id,
+			model.PERMISSION_USE_GROUP_MENTIONS.Id,
 			model.PERMISSION_EDIT_POST.Id,
 		},
 	}
@@ -239,7 +241,6 @@ func TestDoAdvancedPermissionsMigration(t *testing.T) {
 		assert.Nil(t, err)
 		assert.Equal(t, role.Permissions, permissions)
 	}
-
 	// Add a license and change the policy config.
 	restrictPublicChannel := *th.App.Config().TeamSettings.DEPRECATED_DO_NOT_USE_RestrictPublicChannelManagement
 	restrictPrivateChannel := *th.App.Config().TeamSettings.DEPRECATED_DO_NOT_USE_RestrictPrivateChannelManagement
@@ -300,6 +301,7 @@ func TestDoAdvancedPermissionsMigration(t *testing.T) {
 		},
 		"channel_admin": {
 			model.PERMISSION_MANAGE_CHANNEL_ROLES.Id,
+			model.PERMISSION_USE_GROUP_MENTIONS.Id,
 		},
 		"team_user": {
 			model.PERMISSION_LIST_TEAM_CHANNELS.Id,
@@ -420,6 +422,7 @@ func TestDoAdvancedPermissionsMigration(t *testing.T) {
 			model.PERMISSION_MANAGE_OTHERS_SLASH_COMMANDS.Id,
 			model.PERMISSION_MANAGE_INCOMING_WEBHOOKS.Id,
 			model.PERMISSION_MANAGE_OUTGOING_WEBHOOKS.Id,
+			model.PERMISSION_USE_GROUP_MENTIONS.Id,
 			model.PERMISSION_EDIT_POST.Id,
 		},
 	}