Differential Updates View

Summary:
This adds a new view to differential called Updates.

The high-level goal of Updates is to enabled differential to be
effectively used without email notifications. I've tried doing things
like automatically deleting differential emails where I'm in the 'to'
line since they show up on the main diffential page but then there's
always the chance an important diff flies by without me seeing it. Also,
sometimes someone comments on a diff post-commit but differential
doesn't surface those diffs.

I re-created a test db on my devserver using mysqldump to get data on
revs > 230000 so I would have some test data. We need to add a simple
viewtime table but I didn't want to do that in production. Here's the
table:

CREATE TABLE differential_viewtime (
  viewerPHID varchar(64) not null,
  objectPHID varchar(64) not null,
  viewTime int unsigned not null,
  PRIMARY KEY (viewerPHID, objectPHID)
);

Issues:

-Once we turn this on, all diffs will be 'unviewed'. What do you think

about a 'Clear All' button or something?

-Maybe we should add a pager

This feature would be insanely useful, let me know what you think.

Test Plan:
Loaded Updates in my sandbox

http://phabricator.dev1577.snc6.facebook.com/differential/filter/updates/

Clicked a diff, then went back, made sure diff disappeared from Updates
list

Reviewed By: tuomaspelkonen
Reviewers: epriestley, jungejason, tuomaspelkonen
Commenters: epriestley
CC: epriestley, elynde, tuomaspelkonen
Differential Revision: 169