Previously, our Github authentication backend just used the user's
primary email address associated with GitHub, which was a reasonable
default, but quite annoying for users who have several email addresses
associated with their GitHub account.
We fix this, by adding a new screen where users can select which of
their (verified) GitHub email addresses to use for authentication.
This is implemented using the "partial" feature of the
python-social-auth pipeline system.
Each email is displayed as a button. Clicking on that button chooses
the email. The email value is stored in a hidden input above the
button. The `primary_email` is displayed on top followed by
`verified_non_primary_emails`. Backend name is also passed as
`backend` to the template, which in our case is GitHub.
Fixes#9876.
Shubham Padia