Add tests for non-ASCII characters in comments

Due to a bug[1] in ActiveRecord's JSON serialization, tests for characters
outside the BMP are marked as pending.

[1]: https://github.com/rails/rails/issues/3727

set up i18n in comments service

* add i18n gem
* configure default_locale (en-US)
* externalize strings
* configure locale fallbacks
* add tests

Fail fast when database is unreachable

use rack-timeout 0.1.0beta3

fix syntax problem in unicorn config

disconnect default mongoid session after forking.

adjust new relic method tracing.

Fix Content-Type header

The default Content-Type is text/html, but all of our endpoints return
JSON, so Content-Type is now correctly set to application/json.

(also provide a switch for query-level debugging output from moped.)

optimize request timeout settings for heroku

* use Rack::Timeout, set to 20s
* set unicorn timeout to 25s

Ideally these are set lower, around 10s/15s, however we need to reduce long-running requests (such as notifier batch pulls) before we can safely do this.

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.