Clean up Maniphest transaction rendering a bit more

Summary:
Ref T2217. This partially retreads the ground from D7115.

  - We're rendering silly transactions about descriptions when creating tasks. Hide those.
  - Move the "created" transaction back to status. This fixes two things that are otherwise more of a mess than I'd anticipated:
    - It fixes Reports without making a mess (see <https://github.com/facebook/phabricator/issues/395>).
    - It renders old transactions properly (i.e., "created" instead of "reopened" for tasks older than the migration).
  - Be explicit about action strength, so emails always say the most important thing in the subject.

Test Plan: Created and edited tasks, looked at resulting transactions, saw a cleaner transaction record.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2217

Differential Revision: https://secure.phabricator.com/D7141