Search API

[smashwilson]

API for consuming search across indexed documents. The next part of deconst/deconst-docs#187.

• [x] Generate a 400 response if no query term is provided.
• [x] Include titles and body excerpts.
• [x] Test the service with remote storage enabled and ensure that my mocks are consistent with the real Elasticsearch API.
• [x] Include the total result count in the results.
• [x] Update API documentation in the README.