Rewrote MsGraphAdmin token methods

[ChrisToxz]

Had issues with getting admin tokens and sending mail and noted a few mistakes during debugging.

• getAccessToken() returned a redirection to the oath login page instead of getting a new token in the background.
• Oath returns the expiration time as duration rather than a timestamp, this is now stored properly

Not sure if it breaks any other parts of the package. I didn't change any methods names and everything is pretty much in the same flow.

[dcblogdev]

Apologies for taking so long to look at this. I'm running a few tests for this.