Sharing: add Sender data to share by email

This makes conversations a bit easier, since users can now reply to a shared email
It also removes the default
From: WordPress <WordPress@domain.com>
Revert turning off AtD for trunk.

@azaozz has already added a stub to WordPress core trunk to keep this (and other similar plugins) from crashing JS in the browser, so it should be safe to revert the short-circuit.

Reverting https://github.com/Automattic/jetpack/commit/72809cad657dc57ff18aeaa3fed1b28c07a2c8bf
See #38
Jetpack: SSO: Option to remove form now prevents POSTing to form as well
Jetpack: SSO: Allow site admin to automagically forward wp-login.php to WordPress.com login

Introduces the new filter of jetpack_sso_bypass_login_forward_wpcom that can be passed
a boolean. When true users will automagically be forwarded from the wp-login.php page
to WordPress.com to login. Props lessbloat
Merge pull request #59 from jeherve/1745434-sender-share-email

Sharing: add Sender data to share by email