Improve the OAuth dialog

[jdgeenen]

Is your feature request related to a problem? Please describe.
Improve the OAuth dialog.

Describe the solution you'd like
A cleaner dialog that displays the current URL so users have more confidence in how their credentials are handled.
Look into Thunderbird's OAuth dialog for Gmail.

Additional context
https://superuser.com/questions/1430046/how-can-i-know-for-sure-that-this-google-sign-in-is-legit/

[jdgeenen]

mailnews/base/util/OAuth2.jsm appears to be what is used by TB.
It uses Services.ww.openWindow

requestWindowURI: "chrome://messenger/content/browserRequest.xul",
requestWindowFeatures: "chrome,private,centerscreen,width=980,height=750",

and a wrappedJSObject with several members (account, url, iconURI, cancelled, loaded, etc.) and the following: