Principal pre-qualification

I originally thought the pre-qualification form was going to be on it's own page so this will have to be a WIP for the time-being.

We'll have a discussion on Monday to see how best to get this folded into master.

Of importance in this PR is the approach to acceptance testing which includes the use of Page Object Models. The general approach is described [in this post](https://about.futurelearn.com/blog/how-we-write-readable-feature-tests-with-rspec/).