Allow more endpoints to use commentable_ids param

The commentable_ids param is a comma-delimited list of ids indicating
that only threads belonging to one of the specified commentables should
be returned. It can now be used by any endpoint that delegates to
handle_threads_query (most importantly "#{APIPREFIX}/threads"), and
the commentable_id field is now indexed for efficient filtering. The
motivation for this change is to allow clients that are filtering by
commentable_ids to avoid using the /search/threads endpoint, which does
not do sorting and pagination correctly.

Feature/jsa/track slowdowns

Feature/jsa/more new indexes

* use Content model (to avoid querying mongo with "$in")
* add indexes for non-default sorts
* streamline api code

less user queries

performance.

Add newrelic_moped

This will hopefully give us more insight into mongo performance.

This corresponds to the Gemfile change in a9d281ad.

Allow authentication by HTTP header

Previously, authentication could only be done using a URL parameter,
which appears in various logs. Now, clients can authenticate using a
more appropriate HTTP header.

update to latest newrelic rpm

Revert "Merge pull request #29 from edx/chore/markchang/replace-rvmrc"

This reverts commit d97009d1, reversing
changes made to a9d281ad.

moved to more standard .ruby-gemset and .ruby-version from .rvmrc

Update Gemfile

use ThreadPresenter to render search results

Feature/jsa/new handle threads

additionally, remove unused caching features and update mongo indexing.