Search v1.0: Thread, message search for a namespace.

Summary:
1. Script to create an Elasticsearch index for each namespace and index its Threads, Messages
s.t. they can be queried on both thread/message attributes.

2. DSL query constructor to construct Elasticsearch DSL queries for
* no restrictions/ field-restricted/ all fields
* exact phrase/ any match
* equally weighted/ boosted
searches.

3. API functions.
4. Docs.

Test Plan:
Tests for:
Index creation.
Thread, message search.
Cross-attribute search.

Reviewers: emfree

Reviewed By: emfree

Differential Revision: https://review.inboxapp.com/D672