WebHelp: date field in easy search

* Adds "Date: ... until: ..." in the easy search form.
* Builds the range query using invenio sintax "->" because
  the extremes of the range must be included (inspire syntax
  excludes them).

Signed-off-by: Federico Poli <federico.poli@cern.ch>
BibHarvest: description of the attached file for PoS changed

Signed-off-by: Georgios Papoutsakis <georgios.papoutsakis@cern.ch>