[inventory] filtering on dates with $gt doesn't work when supplying datetimes

In order to support device activity checks also for larger device counts the device activity aggregation should be done by the backend/ database (instead of the gui). In order to do so the devices with an updated_ts larger/ newer than the filtered for datetime should be returned along with the total number of devices that match this filter criterion.

Currently the input values for the filtering endpoint seem to not be treated as datetime although they might be valid datetime values.