feature #1907 [Site] Social metadata (smnandre)

This PR was squashed before being merged into the 2.x branch.

Discussion
----------

[Site] Social metadata

Add  FB/Twitter metadata, link the twitter acount and update demo images.

Social images for ackages will be done later

--

TODO After
- [x] bigger social demo images (1200x675) -> will oversize them via Pixelmator
- [ ] one social image per package (on it)

Commits
-------

2a369c6 [Site] Social metadata

Author: kbond