Date selection and enabled buttons. Refs #8244.

- The user can now select today from the calendar.
- I have removed the enable/disable functionality of the not used date picker.
Start date cannot be greater than end. Refs #8244.
Merge remote-tracking branch 'origin/master' into feature/8244_icat_date_selector_issues. Refs #8244.

Conflicts:
	Code/Mantid/MantidQt/MantidWidgets/inc/MantidQtMantidWidgets/ICatSearch2.h
	Code/Mantid/MantidQt/MantidWidgets/src/ICatSearch2.cpp
Prepend catalog helper. Refs #8244.
Merge remote-tracking branch 'origin/master' into feature/8244_icat_date_selector_issues
Merge remote-tracking branch 'origin/master' into feature/8244_icat_date_selector_issues. Refs #8244.

Conflicts:
	Code/Mantid/MantidQt/MantidWidgets/src/CatalogSearch.cpp
Merge remote-tracking branch 'origin/feature/8244_icat_date_selector_issues'