Ajax Signup

Also displays login error message.

Note: I'm not quite sure how to handle registration failure but since this PR generally does the right thing (i.e., doesn't pretend to have created the account, it does a full page reload though). I'm also not sure how to display a success message on registration, although the logged in view of the home page is shown.

Also, the Airbnb modal uses a custom font for the word 'or' which I can't duplicate. This maybe something we change later when a graphics person can be in the discussion.