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)

Co-authored-by: Esteban J. G. Gabancho <esteban.gabancho@gmail.com>
Signed-off-by: Ludmila Marian <ludmila.marian@gmail.com>