Allow projects to own packages

Summary:

• The UI is pretty straightforward, since Handle just works (tm)
• Added two methods to the owners object to handle the new layer of indirection. Then ran git grep PhabricatorOwnersOwner and changed callsites as appropriate.

Sending this to get a round of feedback before I test the non-trivial
changes in this diff.

Test Plan:

• owners tool: edit, view, list for basic functionality.
• phlog for the two new methods I added

Reviewers: epriestley, blair, jungejason

CC: aran

Differential Revision: https://secure.phabricator.com/D2079