Add support for custom date and timestamp formats to reporter and raw data importer

[Nicklas Nordborg]

#1512 added support for timestamps to annotations. A side effect was that extended properties also got support for timestamp properties. The gui has been update to allow for manual timestamp entry for reporters and users, but the reporter importer and the raw data importer can only import timestamps if they are written as yyyy-MM-dd HH:mm:ss and dates if the are written as yyyy-MM-dd. The importers should have the same level of date/timestamp support as was added to the annotation importer.