Start and end date independent of one another. Refs #7871.

- Had to remove check in `CatalogSearch.cpp` as endDate won't appear empty since it's set to the unix epox (1970-01-01 00:00:00) if user doesn't input anything.
- Simplified the query if both start and end date are selected.
Merge remote-tracking branch 'origin/feature/7871_icat_independent_search_controls'