separate Flask app out into its own module

This avoids circular import problems, for example with
bkr.server.systems -> bkr.server.wsgi -> bkr.server.controllers (when
Sphinx autodoc imports bkr.server.systems).

This also lets us avoid importing bkr.server.wsgi which normal code
shouldn't do as it has side effects. That module is really only intended
to be the WSGI application entry point.

Change-Id: I415597700f7c055349d37fb5618d17e658a65863
Merge "separate Flask app out into its own module" into develop