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