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.

Related changes:

- [ ] fix and merge https://github.com/alphagov/static/pull/335 after this one is merged