Naming convention not used when running develop db init

[bchopson]

GH-123 adds a db_before_import signal for defining a naming convention. However, develop db init doesn't use the naming convention defined in the signal handler.

[guruofgentoo]

A common practice is to attach signal handlers in a separate module, and then list that module in the app's visit_modules. However, the database layer gets set up very early in app init, since it can be useful in just about every other aspect of the application. As a result, db_before_import happens long before visit_modules is processed.

Instead, use db_before_import somewhere that gets loaded at import time for the app (e.g. in the module containing the app itself, or something it imports).

Need to put these notes in the docs for that signal. Also should see if there's a way to know if the signal has been called already.

[guruofgentoo]

Added the above notes in the signals docs. There's not really a good way to know if the signal has already been called for an app, and connecting a handler isn't directly related to the app either (the handler just gets called with the sending app when the signal is triggered). I'll leave it at docs for now.