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

Fixes #906 

Sort of a hack, but I didn’t want to break existing functionality by preventing the app from continuing if the OAuth2 login fails.

I'm concerned that we share WPAccount between the WP.com Blog entries and any self-hosted blog as jetpackAccount.  We allow users to edit the Jetpack credentials via "Edit Blog" and this could potentially hose the WPAccount associated with the WP.com Blogs.  

@koke @xtreme-rebecca-putinski - you may wish to review this one in addition to whomever else has dealt with Jetpack in the app.