Load a (currently mocked) hierarchical search response in to a tree (accessible through a root `TreeNode`).

Also, deleted unused class `RefinementList`.
Hiearchical facets are now off by default (even in dev mode), until such time as the back end is ready.
Only mocking geonetwork search requests (instead of all proxied requests).
More consistent pattern for feature toggles.
Merge pull request #1340 from aodn/load_hierarchical_search_response

Load a (currently mocked) hierarchical search response in to a tree (acc...