Improve UX for repository updates

Summary:
Fixes T5926. Fixes T5830. Ref T4767. Users currently sometimes have a hard time understanding repository update frequencies. This is compounded by aggressive backoff and incorrect backoff while importing repositories.

• Don't back off while importing repositories. This prevents us from hanging at 99.99% for inactive repositories while waiting for the next update.
• Back off less aggressively in general, and even more gradually during the first 3 days. This should make behavior around weekends better.
• Show update frequency in the UI.
• Provide an explicit "update now" button to call diffusion.looksoon in a more user-friendly way.
• Document how backoff policies work and how to adjust behavior.

Test Plan:

• Ran bin/phd debug pulllocal and verified backoff worked correctly from debugging output.
• Clicked "Update Now" to get a hint, reloaded page to see it update.
• Read documentation.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4767, T5830, T5926

Differential Revision: https://secure.phabricator.com/D10323