Add support for custom links on top bar

I'm reposting this PR agains the rails3 branch (see https://github.com/puppetlabs/puppet-dashboard/pull/186).

This commit allows a dashboard administrator to add links that point to a specif page on the dashboard that is frequently used. 
In our workflow, when a machine goes through maintenance, we add it to a specific group in the dashboard ('broken_machines'). It turned out that our sysadmins spend a lot of time looking into this specific group, so we thought of adding a link on the top bar to get there faster.

Redmine issue : http://projects.puppetlabs.com/issues/19879