Consider adding Geary to core or removing Contacts

I would add Geary to GNOME core. I still notice a few bugs, but software will always have bugs, and it seems good enough now to become GNOME Mail. If Michael Gratton agrees, we could simply rename it to Mail in the desktop file, the about dialog, and the user help. No doubt the design team will propose further changes, but I don't think we need to gate core inclusion on design changes, since it's already a nice GNOME-style app.

CC @mjog