Signin fragment

Round 1 of ID being incorporated into frontend.

Notes:
* This is not e2e. The sign in fragment is not meant to be released as the sign in form is busy being completed
* You need to run this locally through nginx. See nginx/README.md for what you need to do. This is because we use cross domain shizzle
* The JS is using JSDoc after talking to Patrick. It could come in handy when / if we move into the world of the Closure Compiler
* No integration tests are done. These will be done once we have a user flow. i.e. a sign in form

Hopefully we fair well, but might be worth pairing on any problems as it's the first iteration in integrating.