fix search by date using datetimepicker
corection 1771
Set LAYER_PREVIEW_LIBRARY = 'geoext' as default, refs #1528

Also adds a placeholder for that setting in the debian package local_settings.py
Leaflet cannot be enabled as default until there is a sane way to edit styles.
Updated changelog for version 2.4a31
put last modification on less file
Merge pull request #1772 from Geode/mete_page_css

corection 1771