Add an administrative `bin/repository mark-imported` command

Summary:
Ref T4068. In some cases like that one, I anticipate a repository not fully importing when a handful of random commits are broken. In the long run we should just deal with that properly, but in the meantime provide an administrative escape hatch so you can mark the repository as imported and get it running normally.

The major reason to do this is that Herald, Feed, Harbormaster, etc., won't activate until a repository is "imported".

Test Plan:
  - Tried to mark an imported repository as imported, got an "already imported" message.
  - Same for not-imported.
  - Marked a repository not-imported.
  - Marked a repository imported.
  - Marked a repository not-imported, then waited for the daemons to mark it imported again automatically.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, kbrownlees

Maniphest Tasks: T4068

Differential Revision: https://secure.phabricator.com/D7514