Provide a real object ("PhabricatorRepositoryPushEvent") to represent an entire push transaction

Summary:
Ref T4677. Currently, we record individual actions in a push as PhabricatorRepositoryPushLogs, but tie them together only loosely with a `transactionKey`.

Provide a real PushEvent object, and move some of the denormalized fields to it. This primarily just gives us more robust infrastructure for building, e.g., email about pushes, for T4677, since we can act on real PHIDs rather than passing awkward identifiers around.

Test Plan:
  - Performed migration.
  - Looked at database for consistency.
  - Browsed/queried push logs.
  - Pushed a bunch of stuff.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4677

Differential Revision: https://secure.phabricator.com/D8615
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.

{F134864}

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4677

Differential Revision: https://secure.phabricator.com/D8616
Allow users to receive email about pushes via Herald

Summary:
Fixes T4677. Implements a "send an email" pre-receive action, which sends push summaries.

For use cases where features are often pushed as a large number of commits (e.g., checkpoint commits are retained), using commit emails means users get a ton of email. Instead, this allows you to get an email about a push, which summarizes what changed.

Overall, this is basically the same as commit email, but more suitable for some workflows.

Test Plan:
Wrote some rules, then made a bunch of pushes. Got email like this:

{F134929}

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4677

Differential Revision: https://secure.phabricator.com/D8618