Admin login fails if Google Drive permissions have been granted for the same Google user

Google login to the admin pages at https://lms.staging.hypothes.is/admin breaks if the same Google account used for the login has previously been used to authorize Google Drive access as part of creating an assignment.

**Steps to reproduce:**

1. Create a Google Drive PDF assignment using the Hypothesis LMS staging app. Grant Google Drive access as part of this flow.
2. Go to https://lms.staging.hypothes.is/admin and attempt to login with the same Google account

**Expected:** Login succeeds
**Admin:** Login fails with 500 error

In this scenario, visiting https://lms.staging.hypothes.is/admin redirects to a Google login page with a URL that includes `openid`, `userinfo.email` and `userinfo.profile` scopes. After approving access, Google redirects back to the LMS app at https://lms.staging.hypothes.is/googleauth/login/callback with an authorization code. In the redirect URL the scopes include the requested ones but also have the existing Google Drive scope _added_.

The presence of these existing extra permissions causes an exception on our end ([Sentry](https://hypothesis.sentry.io/issues/6001427034/?project=259908&query=is%3Aunresolved%20issue.priority%3A%5Bhigh%2C%20medium%5D&referrer=issue-stream&stream_index=0)).

A workaround is to go to your Google account settings and revoke Google Drive permissions for the Hypothesis LMS app under security settings.