search: Proper handling of return key in simple search

* Fixed an issue where pressing the return key in the simple search
  dropdown caused the form to be submitted with no data. (#2253)
* Brought functionality into line with that of the advanced search
  tab.
* Form.js could do with refactoring at some point since the simple and
  advanced form methods are not symmetrical.

Signed-off-by: Joe MacMahon <joe.macmahon@cern.ch>