Provide an "event" page for push logs, which shows details on all events in a given push

Summary:
Ref T4677. This shows a more detailed view of an entire "git push", "hg push", or "svn commit".

This is mostly to give push summary emails a reasonable, stable URI to link to for T4677.

Test Plan:

• Pushed into SVN, Git and Mercurial.
• Viewed partial and imported event records.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4677

Differential Revision: https://secure.phabricator.com/D8616