Allow bulk edits to be made silently if you have CLI access

Summary:
Fixes T13042. This hooks up the new "silent" mode from D18882 and makes it actually work.

The UI (where we tell you to go run some command and then reload the page) is pretty clumsy, but should solve some problems for now and can be cleaned up eventually. The actual mechanics (timeline aggregation, Herald interaction, etc.) are on firmer ground.

Test Plan:

• Made a normal bulk edit, got mail and feed stories.
• Made a silent bulk edit, no mail and no feed.
• Saw "Silent Edit" marker in timeline for silent edits:

{F5386245}

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13042

Differential Revision: https://secure.phabricator.com/D18883