Manage sections and modules

This PR adds two new tabs to users with the correct permissions - one to manage modules, and one to manage sections. There are two seperate permissions, for managing sections, and for adding sections. 

Once the sections have been added, things like the hero images and links can be managed, as well as what modules will be on the section homepage, as well as the ordering (via drag and drop). 

Basically, everything that's managed via the Yaml file, but without the Yaml.

Needed for theodi/shared#137