Style corp information page language links

This extracts the styling for .available-languages into
a css helper, taking it out of the heading scope.
Add additional fractions tests
Always render using html fractions partial
Merge pull request #1589 from alphagov/fix_fractions_in_atom_feeds

Fix atom feed where fractions are present in a publication
Merge pull request #1588 from alphagov/fix-translation-links-on-corp-info-page

Style corp information page language links
Adding additional fractions assets
Updating fraction symbol with improved artwork
Running images through ImageOptim
To compress and optimise cross browser
Uppercase versions of 1/y and 1/x to meet syntax needs
Handle Invalid URIs in LinksChecker

URI.parse will fall over if the provided URI is invalid. This adds error
handling to LinksChecker so badly formed URIs do not cause the checker
to fall over.
Merge pull request #1591 from alphagov/handle-bad-uris

Handle Invalid URIs in LinksChecker
Add data migration to redirect MMO content

The Maritime Management Organisation have unpublished and deleted a large amount of their content, publishing replacement content at other URLs.

This migration updates the Unpublishings for the content, so that they 
refer to their new locations.
Merge pull request #1590 from alphagov/additional_fractions

Additional fraction images

https://www.pivotaltracker.com/story/show/74213934
Adds algebraic and trigonometric fraction images requested by DfE
Merge pull request #1592 from alphagov/add-redirects-to-mmo-unpublishings

Add data migration to redirect MMO content

https://www.pivotaltracker.com/story/show/74448752
Fix broken link on Readme.md

Changed contribution guide to point to the file that is now in the main directory rather than the docs folder.
Merge pull request #1593 from kyconny/patch-1

Fix broken link on Readme.md