Allow syncing with Google Drive

[aecreations]

I experimented with integrating Read Next with Google Drive, and can confirm that the Google Drive API works when called from a Firefox WebExtension.

Proceed with implementing this for an upcoming release.

[aecreations]

Need to fix CORS error that sporadically occurs when attempting to call the API to get the sync file's last modified timestamp:

Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at https://www.googleapis.com/drive/v3/files/<sync file ID>?fields=modifiedTime. (Reason: CORS request did not succeed). Status code: (null).

[aecreations]

The above noted error tends to occur when my computer is asleep; it's likely that Read Next is trying to call the API just when the computer is re-initializing network connections upon resumption from sleep mode.

The MDN documentation says that: "The error is not directly related to CORS, but is a fundamental network error of some kind."[1]

The error goes away when Read Next performs another automatic sync.

--
[1] https://developer.mozilla.org/en-US/docs/Web/HTTP/Guides/CORS/Errors/CORSDidNotSucceed

[aecreations]

The sync file is moved from the hidden app folder to the user's root folder in Google Drive, to ensure its visibility to the user.

Confirmed that if the sync file is moved to a folder, Read Next will still be able to locate it by the file ID.

[aecreations]

Detect if the user didn't grant Read Next sufficient permissions to Google Drive. This can occur if the user didn't tick the checkbox during the authorization flow:

If this happens, display an error message and a "Disconnect" button in the connection wizard:

Connect to Google Drive

Read Next can't sync your reading list because it wasn't given enough
permission to access Google Drive.

< Disconnect >