Editable footer links

This adds a basic function to allow an admin user to hide and show the links for the static pages that are in the footer. 

For example, the admin can make the "About Us" link visible.They visit /pages/about/edit , check the "Show a public link to this page" checkbox, then save/update the page.  Similarly, they can hide the footer link to "404".
![link_visible_screenshot](https://cloud.githubusercontent.com/assets/2813357/2842409/87d73960-d074-11e3-8d79-45f9e792fda6.png)

Note that the order the links appear in the footer is undefined. This is a regression of normal but unspec'ed behaviour so please ask me to add a feature flag (or postpone the pull request) if such a regression would be undesirable. (A simultaneous pull request sorts the pages in the index alphabetically, but does not affect the links.)

It is staged at http://localsupport-northumbria.herokuapp.com/

It implements the first part of the functionality requested in the user story "I want to be able to edit static page links" https://www.pivotaltracker.com/story/show/52536437

Future pull requests may improve the UI, and add a function for the admin user to customize the order the links display in, ~~and automatically create a permalink slug.~~

**Requires `bundle exec rake db:migrate` to add a new database column `link_visible`. The migration automatically leaves a '404' static page link hidden, if you have one.**

Note: I also moved some cucumber step definitions from map_steps.rb and navigation_steps.rb to the the new file, static_pages_steps.rb which contains new definitions I needed for this feature.