Have an easy to use way to input user details

[nrc]

We should have a way to create the initial entry for a user without them going through the fork and submit a PR dance.

The work flow should be something like - login using GitHub auth, fill out a form on rustaceans.org (username should be supplied from the auth), create a json file from the form details, submit a PR of this json file to the rustaceans repo using the user's credentials. I think this is all possible using the GitHub API, and we probably don't need to actually have a copy of the repo on the server, we ought to be able to synthesise the PR.

On success, the user should get notified and get a URL for themselves on rustaceans and a URL to their data entry in the repo, and a URL and instructions for how to submit a PR to change their details.

(I'm not too concerned about having an interface to edit a user's details, although that is hopefully possible too - the rate of change is very low, and doing a PR is not too hard).