introducing an index page for top contact reasons

(this pull request replaces #79)

Analysis of contacts from GOV.UK users has shown that most users get in touch
because of specific transactions and services, rather than anything to GOV.UK
itself.

To provide users with a more direct contact route for the most common issues,
this change introduces a new index page with links.

Additionally, this change updates the URL structure to take into account that
most users don't actually want to leave feedback, but rather to contact us.