Fix links in header

- "G-Cloud Submissions" should be highlighted, i.e. it should be
  evident what page of the Marketplace you are on
- The service name ("Digital Marketplace") should link back the
  Digital Marketplace homepage, not the submissions homepage
Make Digital Marketplace not link to Submissions
Merge pull request #89 from alphagov/fix-header-links

Fix links in header-tested and passed on QA environment