WebSearch: parenthesised collection definitions
WebSearch: parenthesised collection definitions

* Allows collection definitions to use parentheses.  This is useful
  for complex (virtual) collection definitions for which usual
  left-to-right expression combination is not enough.

* Cherry-picked from 521854eec672d36fb74f17c982953b7b461d87b5.

tiborsimko/invenio-search,tiborsimko/invenio-records,inveniosoftware/invenio-search,tiborsimko/invenio-search,inveniosoftware/invenio-records,inveniosoftware/invenio-search,inveniosoftware/invenio-records,tiborsimko/invenio-records