In order to properly deploy phabricator with scap3, /srv/phab needs to link to the scap3 deployment repository.

Note: Need to ensure that the backup job is updated to look for /srv/repos instead of /srv/phab/repos

Steps:

1. wait for maintenance window
2. Stop puppet, apache2 and phd on iridium
3. merge https://gerrit.wikimedia.org/r/#/c/274484
4. move /srv/phab/repos to /srv/repos
5. T125851: Refactor phabricator module in puppet to remove git tag pinning behavior (patch submitted, pending merge)
6. rename /srv/phab to /srv/old_phab
7. create a symlink named /srv/phab pointing to /srv/deployment/phabricator/deployment
8. test-run with puppet to make sure that it behaves
9. double-check that /srv/phab is now pointing to a sane phabricator/deployment tree with the correct tags checked out
10. start puppet, phd and apache2