Define search view via a setting.

[cmc333333]

To use the elastic-based search, users had to write a new urls.py module (and intuitively know that es_search was an option). Allowing this to be configured (and documenting how) will be helpful while we work on a postgres-specific search.

[coveralls]

Coverage increased (+0.004%) to 92.986% when pulling efe6676b584896387cc2f6ae7f3ee2b54840415b on 475-configurable-search into 7819155d4530f67d6fc9162f46b297a4a6794221 on master.

[cmc333333]

Thanks @tadhg-ohiggins