Connect widget signals during RB initialization instead of module load.

reviewboard/admin/__init__.py was connecting to save/delete signals for
Repository and Group in order to invalidate widget caches, in order to
keep the dashboard up-to-date. However, doing it during module load
resulted in some circular import issues, which manifested in various
ways over time, but are now impacting the docs generation.

Now, the signal connections and invalidation live solely in widgets.py,
in an init_widgets() function.

The top-level reviewboard.admin module listens for Review Board's
'initializing' signal and, when called, it invokes the init_widgets().

This allows us to build docs again, and should resolve some of the other
cases we've seen over time.

Reviewed at https://reviews.reviewboard.org/r/6370/