Rearrange __init__.py so common code is local

I've moved all the importing (bar the loading of routes) into one place
as well as batching up the setting of app information.
Add Sentry to admin app

We are adding this so we have a central place we can go and look at our
exceptions. This will only sent stuff back to sentry when the SETNRY_DSN
is set, but shouldn't interrupt our app if it isn't.
Merge pull request #40 from alphagov/add-sentry-tracking

Add sentry tracking
Need to get app.config before setting up Sentry

Sentry requires a var off of app.config, so it was a little silly to try
and instanciate it before having loaded the config.
Remove redis session

Managing token caching and reauth has been moved back into stagecraft to
avoid a call back to signon for every request there.
Merge pull request #42 from alphagov/add-sentry-tracking

Need to get app.config before setting up Sentry
Log user out on 401 not 403

Unauthorized and Forbidden have been split out in stagecraft now. We
should not sign the user out if they do not have sufficient permissions.
Merge pull request #41 from alphagov/remove-redis-session

Remove redis session
Alphabetise requirements

because pedantry
Merge pull request #43 from alphagov/sort-requirements

Alphabetise requirements