Fixes bug 1013321 - Admin UI for the master list of Super Search Fields

This does three important things:
1. Move the fields from a JSON file to elasticsearch, so they can be modified.
2. Add middleware services to add, edit or delete fields.
3. Add an admin UI to view, add, edit or delete fields. 

Along that, it also adds a way to refresh the cache in the webapp models. 

@peterbe r?

Apologies for the, once again, big PR, but all those things are quite tied together, so it would have made little sense to split them into smaller PRs. 
The error handling in this work is very poor. The main reason is that I don't really know how to do it, and I don't know if we have good examples or utilities already in place in the webapp. The second is that I thought it was good enough for now, especially for a UI that will be used by at most 5 different people. 

If you have good resources about how to best handle errors in django, I would be very interested.