Add GitHub as an additional allauth authentication backend.

- [x] Add custom allauth provider
- [x] Extend signup form to prompt user to chose which email address to use
- [x] Apply frontend changes requested by @stephaniehobson in bug 1036140
	- [x] Unverified email addresses will trigger the MDN email verification process.
	- [x] If the user does not have an email address we don't have to show the radio buttons just the text field
	- [x] move the "Having trouble logging in?" link down the page
	- [x] make all the headings look like the newsletter one
- [x] add missing initial connect to persona on profile form
- [x] replace all "e-mail" with "email"
- [x] check if persona still works with github-login disabled