Add more social media services

We add: Pinterest, LinkedIn, Foursquare, Google Plus, Email, Other.  We've added their icons to the sprite used for the frontend too.
Hide social media accounts behind a +others link if there are > 5
Add administering social media scenario to the orgs feature
Allow for an optional title when defining a social media account
Use the title if present when showing social media accounts

Fall back to the service name.  The text will then be a bit odd "Connect with <org name> on @fco-birthday-tweets" - but it might be ok because we hide the "Connect with <org name> on" anyway.
Extend social media scenarios to cover specifying a title
Merge pull request #415 from alphagov/more-social-media-account-functionality

More social media account functionality

https://www.pivotaltracker.com/story/show/41124335