use a single index in elasticsearch for both document types.

WARNING: Must rebuild search index!

Starting with this change, the name of the search index used by the
service has changed.  The new search index must be created before
deploying this version of the application.  There are two ways of doing
this:

* Offline (recommended)
Follow [these instructions](https://github.com/edx/cs_comments_service/wiki/Search-Indexes#offline-procedure) to perform an offline rebuild.

* Online
1. Deploy this code to a host which is not serving HTTP requests, and
run `rake search:rebuild`
2. When the rebuild finishes, deploy the updated code on live servers.
3. run `rake search:catchup[MINUTES]` where minutes is the amount of
time it took to run rebuild in step 1.
4. run `rake search:prune`