Allow Default Reviewers to be bound to one or more repositories.

Our Default Reviewer code pre-dated our multi-repository support, and was
never updated to work with repositories. In most cases, a default reviewer
path really makes sense only in the context of a single repository, or maybe
just a couple.

This change allows a Default Reviewer to be bound to one or more repositories.
A Default Reviewer that isn't bound to any particular repository will
continue to work with all repositories, partly for backwards-compatibility,
partly for the maintenance burden on the administrator's behalf.

As part of this, I've cleaned up the Default Reviewer administration page:

* The Name and Regex fields are now longer.
* Regular expressions are validated before saving.
* The Default Groups field is now a smart side-by-side add/remove field
  instead of a basic multi-selection field.


NOTE: This is a migration to Git for /r/981/. See there for screenshots.

Added unit tests for the repository query code, which I've verified works fine. This query code handles checking for default reviewers that either match the given repository or don't specify any repository (therefore working with all).

Tested the regular expression validation.