Improve how indexes are created/handled

[stefandesu]

Eventually, it should be able to create and update indexes via HTTP endpoints instead of only via the import script (continuation of #101):

• Create service methods for creating indexes
• Figure out who can create indexes via endpoints and how this is configured
• Create endpoints for creating indexes
• Documentation

It might be possible to handle indexes much more elegantly via our models, so it might make sense to look into that.