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:
  
• 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 issue appear as expected.
• Ran ./bin/config done reindex and observed reindex issue cleared as expected.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T12164

Differential Revision: https://secure.phabricator.com/D19497