Support for OR Logic Conditions in Client API

[prof-m]

In the Client API's /get_files/search_files endpoint, it'd be great if we could add support for OR conditions when searching for multiple tags. That would let API consumers make fewer API requests when trying to get a result set that contain optional tags, and allow for Hydrus to do the sorting on the results before they're returned to the API consumer (once the API supports sorting).

[floogulinc]

Added in https://github.com/hydrusnetwork/hydrus/releases/tag/v453