This introduces a new set of standardized condition choices and
operators for any extensions, integrations, or built-in UI that wants to
offer user-customizable condition sets based on review requests,
repositories, or review groups.

The repository-based condition choices/operators allow for matching
repositories based on the SCMTool type, the public/privacy level,
set/unset state, or by comparing to a list of user-defined repository
entries.

The review group condition choices/operators allow for matching a
list/queryset of review groups based on the invite-only flags,
empty/non-empty states, or by comparing to a list of user-defined review
group entries.

The review request condition choices/operators build upon both of these,
offering those choices in relation to the review request's target group
list and associated repository. It also includes operators for built-in
fields and for matching modified files in the current diff.

Unit tests were added that cover everything but the most trivial
choices (those that are simply based on already-tested choice classes in
Djblets).

Unit tests pass.
Made use of these in an integration. From current testing, they work

as expected (though more thorough testing will be needed for corner

cases).