reviewboard.site had previously started listening to signals in two

places:

The top-level reviewboard.site module, for managing OAuth2

   application access.
reviewboard.models, for triggering a local_site_user_added

   signal.

The OAuth2 one probably shouldn't be in this app, and in fact it also

listens to a signal (initializing) to set up webapi scopes.
Since this is a mess, and more signal handlers will soon be coming as

part of another change, this change takes the opportunity to clean this

up.
New reviewboard.site.signal_handlers and

reviewboard.oauth.signal_handlers modules have been added, each with a

central connect_signal_handlers() method.
The one in reviewboard.site sets up the trigger for the

local_site_user_added signal.
The one in reviewboard.oauth sets up the application access management

and enables scope.
Both apps now have an apps.py, which calls

connect_to_signal_handlers() when the app is ready. This means we're

not setting up on initializing, which currently happens on first HTTP

request. Instead, we're setting up when the app is ready, meaning we can

be better prepared before HTTP requests are handled.

Unit tests pass.