Preload Whitehall in production

The time before the app begins serving requests is currently quite large
due to the way Rails lazily loads its object graph. During this time it
times out frequently while it warms up.

This commit tells Unicorn to preload the entire application, and
includes the standard database connection futzing that seems to be
common to these solutions.
Merge pull request #583 from alphagov/preload-app

Preload Whitehall in production