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. 
Call that `HERE` instead or `THIS_DIRECTORY`. 
Also, consider calling it `_HERE` (or `_THIS_DIRECTORY`) or even `_this_directory` because it's not a traditional "constant". Constants are often written in all caps but that's because you might potentially change them or at least indicate that it's a human choice at play. E.g. `PI = 3.14`.
What you're using for here is just a variable so you don't have to type `os.path.dirname(os.path.abspath(__file__))` in a bunch of places in the code.