Solr search

Solr integration (using spring-data-solr). This feature push introduces the following:
* Adding drops to the search index once they've been stored in their respective tables
* A `search/` endpoint to handle all search functions - drops, users, buckets and rivers. This endpoint is unauthenticated and only returns the publicly accessible info
* Separate spring beans configuration file for the solr server

For testing/dev, use embedded solr search server but for production, use an external Solr HTTP Server