Validate and refresh OAuth access tokens to ensure the user can utilize cloud storage

[linuxwolf]

In service to sync, timely updates of the account access rights are necessary. This involves verifying the OAuth access_token is present, current, and active by requesting current user profile information.

This is exposed to the rest of the extension internals as a token() method, does the following:

• checks for the presence, expiration, and activeness of an access token

• refreshes the access token if possible

• caches updated access token, expiration time, and profile information

  • OPTIONAL alerts the rest of the extension of updated account details

• returns the access token on success, or throws a AUTH error on failure

Checking the access token involves the following, marking for refresh if any fail:

1. checks for the presence of a cached access token and expiration time
2. checks the expiration time for the access token
3. requests the user's current profile information to confirm the access token is still active

Refreshing the access token involves the following, throwing an AUTH error if any fail:

1. Checks for the presence of a cached refresh token
2. makes an OAuth token exchange request using the refresh token
3. checks the newly acquired token is present, current, and active (as above)