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