Set extension access security for non-Chrome logins.

The current security model for the extension assumes that the student is logged into both a google account linked to the class and logged into Chrome with the same account. While this is the norm for chrome users it is not for other platforms. Ergo we will need to consider another security model down the road.

No files, tests, or entry points are named. Start by documenting how access currently depends on a Google account linked to the class and the matching Chrome login, then define the requirements for a non-Chrome security model. Done means an agreed alternative model and implementation scope are recorded.

Written by the indexing model from the issue text.