Don't show the profile social tools on mobile

closes #528
Add social sharing to the footer

closes #357
Clarification notes about sorting dates
Add an admin command to fix party membership position end_dates

This is a fix for issue #550 - some party membership positions
were set to 2012, so they no longer appear as current
party memberships.  Instead, we should set these to 'future'.

We will shortly be importing the new candidate data, in which
lots of these will be ended and new party positions will be
created for a new party.
Serve robots.txt from Django so we can vary it depending on settings.STAGING

closes #557