Allow mapping student|device via something other than device location

[jay0lee]

Currently the extension can only read allowed users from the device location field which some admins may be using for other purposes. This decision was made in order to keep the extension very simple and avoid needing to make Internet API calls out to Google. Force-installed extensions can read the current devices Location attribute even when offline.

• Consider reading a custom schema on the user themselves that would contain a list of allowed Chromebook serial numbers / asset ids.
• Reading a user attribute would require an online Google API HTTPS request opening the extension to timing attacks where the user disconnects the device at just the right time in order to cause the extension to fail. Extension would need to catch and retry on failure.