Adds tables for Elasticsearch clusters and indices to the database. Close #1695

The models are not used anywhere yet, but will be useful in #1696. For now, the immediate gain is that admins can see an overview of available indices in the admin site. This is information is also available in Kibana, but it's neat.

Keeping the models in sync is all optional at this point, but can be done as follows:

• There is a django-admin command, update_index_metadata, which will update servers from the Django settings, and indices for each server. I've added this to the yarn start-back command.
• The admin site includes an action to update index metadata per server, or update the availability status of a single index.

Changes for deployment

It's not strictly necessary to change anything, but I recommend adding the update_index_metadata command to the startup routine in the deployment script.