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.

Unit tests pass.
Added some logging to _increment_sync_num, and saw it was being called

when I saved or deleted groups and repositories.