Fixing accessibility issues

• adding in content id and role main to wrapping div
• update h3 to h2 for share this page header to make it sequential
• add id to topics, orgs and worldwide index filter form inputs to fix
broken link between it and it's label

https://www.pivotaltracker.com/story/show/63536308
updating div id main to main id main after pull request feedback
Merge pull request #1199 from alphagov/accessibility-fixes

Fixing accessibility issues

https://www.pivotaltracker.com/story/show/63536308