Changed the contribute button for central Landing Page.

The button used to redirect to GitHub, but now goes to the Development
page of the KA Lite Wiki.
Updated Contribute section text to reflect new ways to contribute.

Now above the contribute button it suggest users to develop, translate,
or seed.
Added social-follow buttons to central landing page.

Previously there were only social-sharing buttons, now social-follow
buttons have been added to the footer.
Changed langselect block name to footerleft for efficiency of use.

Previously there was a langselect block used for the distributed
server, and there was a need to place something in the same spot
on the central server. Therefore it was necessary to make the block
more general, so it was changed to footerleft.
Merge branch 'develop' of github.com:jamalex/ka-lite into develop