WebSearch: resolve (internal) DOIs

* When accessing "CFG_SITE_URL/doi/foo/bar", try to resolve 'foo/bar'
  as DOI and return corresponding record (0247__a:'foo/bar'). In this
  initial release the 'source' field (0247__2:doi) is not considered.
  Closes #1322.

Signed-off-by: Jerome Caffaro <jerome.caffaro@cern.ch>
Reviewed-by: Ludmila Marian <ludmila.marian@gmail.com>