Add icons, colors and action names to Maniphest timeline view

Summary: Ref T2217. These are mostly me making stuff up rather than some bird's-eye view of color and iconography across applications, yell if any of these seem off once this rolls out.

Test Plan:
  - Looked at a bunch of transactions, saw reasonable looking colors and icons.
  - Sent email, saw appropriate subject line actions.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T2217

Differential Revision: https://secure.phabricator.com/D7075