Makes a Jetpack account log into WP.com to get an OAuth2 token

Sort of a hack, but I didn’t want to break existing functionality by preventing the app from continuing if the OAuth2 login fails.
Don’t bother logging in using the shared API in Jetpack UI

The WordPress auth call should have already been done in Blog+Jetpack
Updated comment in Blog+Jetpack for a future TODO
Merge pull request #912 from wordpress-mobile/issue/906-jetpack-oauth2

Makes a Jetpack account log into WP.com to get an OAuth2 token