WebSearch: new Add-to-Search Interface

* New search box interface that contains both simple search and advanced
  search (javascript dependent).  (addresses #622)

* Replaces the 3-line advanced search, with only 1-line add-to-search,
  with the possibility of adding patterns to the current search pattern.

* Ammends regression tests to take into account a third search
  interface.

* Moves the config variables `CFG_WEBSEARCH_ENABLED_SEARCH_INTERFACES`
  and `CFG_WEBSEARCH_DEFAULT_SEARCH_INTERFACES` from the `inveniocfg.py`
  to `search_engine_config.py` and `invenio.config`
  respectively.  (closes #271)  (PR #1738)

Signed-off-by: Ludmila Marian <ludmila.marian@gmail.com>
Co-authored-by: Esteban J. G. Gabancho <esteban.gabancho@gmail.com>
Reviewed-by: Tibor Simko <tibor.simko@cern.ch>