WebSearch: fix legacy search field names feature
WebSearch: fix legacy search field names feature

* Revive the old facility that enabled site to define legacy search
  field names in CFG_WEBSEARCH_FIELDS_CONVERT for the search engine
  to accept as aliases.  (E.g. old legacy alias `au' for `author'.)
  The facility was long unused, hence it worked only partially.
  (closes #292)
  (addresses #283)

inveniosoftware/invenio,tiborsimko/invenio,tiborsimko/invenio,inveniosoftware/invenio