Add minimal admin app

Just add a homepage and allow the app to be started in the same way as
the read and write apis are.

Includes a refactor of the `start.py` script.
Add configuration to admin app
Refactor logging and set up for admin app

- Set the Flask app name correctly for each app
- Use the Flask app name when setting up logging
- Add logging support to admin app
Remove Signonotron2's dependence on flask

The Signonotron2 class doesn't need to depend on flask and it doing so
has the depdency going in the wrong direction.
Add sign in feature to admin app

This includes:
- Copy static assets and templates from the write api
- Add authentication related views
- Add final homepage view

This commit introduces a *lot* of duplication between the write api and
the admin app. The write api is being left as it is until this is merged
to avoid tying the backdrop release to a release of `pp-deployment`.