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/permissions_test.go

@@ -255,7 +255,7 @@ func TestImportPermissions_schemeDeletedOnRoleFailure(t *testing.T) {
 
 }
 
-func TestEmojiMigration(t *testing.T) {
+func TestMigration(t *testing.T) {
 	th := Setup(t)
 	defer th.TearDown()
 
@@ -264,6 +264,7 @@ func TestEmojiMigration(t *testing.T) {
 	assert.Contains(t, role.Permissions, model.PERMISSION_CREATE_EMOJIS.Id)
 	assert.Contains(t, role.Permissions, model.PERMISSION_DELETE_EMOJIS.Id)
 	assert.Contains(t, role.Permissions, model.PERMISSION_DELETE_OTHERS_EMOJIS.Id)
+	assert.Contains(t, role.Permissions, model.PERMISSION_USE_GROUP_MENTIONS.Id)
 
 	th.App.ResetPermissionsSystem()
 
@@ -272,6 +273,7 @@ func TestEmojiMigration(t *testing.T) {
 	assert.Contains(t, role.Permissions, model.PERMISSION_CREATE_EMOJIS.Id)
 	assert.Contains(t, role.Permissions, model.PERMISSION_DELETE_EMOJIS.Id)
 	assert.Contains(t, role.Permissions, model.PERMISSION_DELETE_OTHERS_EMOJIS.Id)
+	assert.Contains(t, role.Permissions, model.PERMISSION_USE_GROUP_MENTIONS.Id)
 }
 
 func withMigrationMarkedComplete(th *TestHelper, f func()) {