[Users] How to configure Gmail with OAuth2

[c. marlow]

I just now sat down and went through the instructions on how to set up
Gmail in Claws with OAuth and the help guide is missing some steps:

Under OAuth credentials

I had to click + CREATE CREDENTIALS and fill that out because I had
nothing listed under  OAuth 2.0 Client IDs at first.

I also had to add my gmail address to the TESTING list.. Otherwise I
got a 403 forbidden error when trying to authenticate the Authorization
code. 

Chris