Add an error message when ES mappings aren't right

The breakage I caused yesterday was probably because I clicked the wrong button to do the re-indexing between steps one and two. Since the new index had only been created, but not set up, it had a default, ES-inferred mapping, which really screws up our stuff.

This adds a function to `es_utils.py` that tries to determine if the mappings for a given index (or all of them) lines up with what the code expects. I then used that to add a management command and a warning message to the ES management admin page.

Thoughts? r?