Strip whitespace from social media urls

We strip the whitespace in the controller before sending it to the
model.

Also includes a data migration to update the existing social media urls
that are broken (27 on this morning's database dump)
Merge pull request #530 from alphagov/strip-whitespace-from-social-media-urls

Strip whitespace from social media urls

https://www.pivotaltracker.com/story/show/49053903