Allow users to unlink GitHub accounts

[outoftime]

Since GitHub login sessions are indefinitely long-lived, it’s common for a student to be on a shared laptop that’s logged in to a different student’s GitHub account. Thus, it’s a common problem for a student to accidentally link a different student’s GitHub account to their own Popcode account. While the “merge accounts” flow includes an explicit confirmation that we’re looking at the right GitHub account, if the erroneous GitHub account is not yet associated with a Popcode account, there is no such confirmation.

So, we should allow students to unlink the GitHub account that‘s linked to their Popcode account, to make it easy to undo this mistake.

From a UX perspective, we will want to show enough information about the linked GitHub account to allow the student to make an informed decision about whether it’s the correct one. So, add a component to the logged-in user drop-down that shows the user’s GitHub avatar, display name, and username, along with an Unlink button. For consistency, we’ll just show the same information about the linked Google account, although there will be no Unlink option here because Google is the primary authentication provider for Popcode.

[outoftime]

Closed by #1637