Credentials for hosting service accounts don't change very often, but
when they do, we don't provide any sane way of resetting/editing them.
The workaround before was to nuke the account data in the database
editor and then re-edit a repository, but this was pretty terrible.

We now have an "Edit credentials" link next to the selected account in
the repository configuration form. When clicked, the auth form will
appear, allowing the user to provide new credentials to use.

The user can cancel editing, or they can save the repository to
re-authenticate.

Tested saving new credentials for an account.
Tested attempting to save bad new credentials, and seeing the error

message (with the form staying open).
Tested saving new credentials that then required a 2FA token (with the

form staying open).
Tested canceling edit.
Tested edit mode and then switching accounts/services.