Extract tag controller methods into shared module

This extracts the methods and actions in MainstreamBrowsePagesController
into a reusable module, and moves the views into a shared directory.

This will allow us to easily add a new type of tag, with similar
behaviour, in a subsequent commit.

I'm not sure if using a module here is the best idea, versus subclassing
a controller (feedback is welcome).
Allow Topics to be created, updated and published

This creates a new controller to create, update and publish topic tags,
using the module extracted in the previous commit.
Update the navigation bar

This updates the navigation bar at the top of the app to include links
to edit 'mainstream browse pages' and 'topics'.

I've removed 'GOV.UK' from the app title to free up some additional 
space in the header.