Major changes to Social network logic

* **Functionality change**: when there's a featured image, output only that for both Twitter and FB, ignore other images in post.
* **UX change**: rework logic for showing networks on Social tab, social network no longer shows on social tabs if not enabled in admin.
* Adds Twitter inputs to the Social tab.
* Always output a specific Twitter title and description, as otherwise we can't overwrite them from metabox.

Props @tacoverdo for input :)