Change LDAP group sync 'Clear LDAP permission cache' to 'Sync Now'

Currently, if LDAP group sync is enabled for a group, masters/owners see a 'Clear LDAP permission cache' button. Now that we have proper group-based LDAP syncing we can sync group membership on-demand. This depends on https://gitlab.com/gitlab-org/gitlab-ee/issues/399 but after that, this should be doable. Change the button to 'Sync now', kick off a worker run, and displaying the syncing status. Displaying the syncing status will be the most challenging piece, probably.

cc/ @jacobvosmaer @DouweM Is there a developer that would be good to work on this?