WebSearch: full-text search optionally via Solr

• When CFG_SOLR_URL is set, use Solr instance to provide full-text search. This solution allows communication with the Solr instance via intbitsets. Note that necessary parts for the Solr instance are not covered by this commit.

• Do not run Invenio's native bibindex for full-text index when Solr is being used for full-text search. Note that currently we rely on the Solr index to be up-to-date; the update of indexes will be covered later.