Add new document button to navigation in admin

Creating new documents is a fairly common action. This should make
doing that action easier and faster for editors.
Remove console log
Use url helper for TopicalEvent#search_index

It was crashing when doing a full index rebuild with:

undefined method `topical_event_path' for #<TopicalEvent:0x0000000df68cd8>
app/models/topical_event.rb:89:in `search_link'
Merge pull request #577 from alphagov/new-document-nav-admin

Add new document button to navigation in admin