Add support for OAuth 2.0 connections to email server

Add the option for configuring this app to use OAuth 2.0 instead of username/password for connecting to the incoming email server.

Currently, this app only supports username/password authentication for connecting to the email server.

Some email servers, like gmail using IMAPS, consider a username/password authentication to be "Less secure app access" even when it is done over SSL. They require enabling "Less secure app access" for connections to be made from this app using username/password authentication.