GMail: Send email with OAuth2

[cfbao]

Provide OAuth2 support for email report for better security. Users won't have to check "Allow less secure apps" in their Google account.

The simplest way to do this is probably via GMail API. I already have some rough code that can send email this way. The downside is that other mail services aren't covered.

A more general way would be to use SMTP, but this seems more difficult.