R1:a6951a0a5aa0

Add migration to encourage rebuilding repository identities

Summary: Ref T12164. Defines a new manual activity that suggests rebuilding repository identities before Phabricator begins to rely on them.

Test Plan:
- Ran migration, observed expected setup issue: {F5788217}
- Ran `bin/config done identities` and observed setup issue get marked as done.
- Ran `/bin/storage upgrade --apply phabricator:20170912.ferret.01.activity.php` to make sure I didn't break the reindex migration; observed reindex setup…