Allow students to link their account to an identifier after they create a repo

[amueller]

Feature request ✨

It looks like it's not possible for students to assign an identifier to their account once the repo is created.
I had several students enroll late to the class. Many of them started the homework, leading to unlinked accounts. Now I need to link each of them individually.

[d12]

Students who did not link will be prompted again when they go to accept a second assignment. It would be nice to have a way for students to re-do the linking process without needing to accept a second assignment though :)

[amueller]

Thanks. That is also not obvious ;)