New words on homepage
Add new about page

- Add new navigation
- Link to new /performance/about resource
- Skeleton HTML for Henry to put stuff in
Fix typo in routing
Update navigation links
Add Rosalie's text for the About page
Add statement about personally identifiable data
Use en-dash
Treating Excel as a noun
Minor style tweak

This is consistent with the style guide usage guidelines.
Remove unnecessary object extend
Add Rosalie's text for the About page
Make about page section width device specific

Also use a Sass calculation for it.
Merge pull request #569 from alphagov/rosalie-words

Add new about page and navigation