Adds new widget 'Subscribe Google Groups', that allows user to enter e-mail address she wants to get subscribed into admin-selected Google Group.

Behind the scenes plugin sends subscription e-mail to GROUPNAME+subscribe@googlegroups.com. If everything goes well, user gets notified for succesful operation. If WordPress detects error while subscribing, user gets error message.

If domain for the subscribing user has published strict SPF/DMARC policies, receiving domain might reject mail sent from invalid server (server hosting the WP site).

Also, see grep -RE "(TODO|FIXME)" ..