Signin form with id api

Sign in form on ID frontend site now works-ish.

* introduces ID Scala API client with pluggable HTTP implementations
* signin form integrated with API client
* update messages and some tidying up of the generated form HTML
* introduces omniture tracking for signin

Also updates Identity's setup:
Identity is no longer a part of dev-build because cookies doesn't work on localhost anyway so it was no use. Identity includes a setup README and its own sbt command `idrun` so it can easily be run on the port nginx expects.

This is the last epic pull request for a bit, subsequent requests will be slimmed down.