`reviewboard.site` had previously started listening to signals in two places: 1. The top-level `reviewboard.site` module, for managing OAuth2 application access. 2. `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.