Replace laws_references with Solr functionality

[waldoj]

Now that Solr is part of the install process, we really don't need laws_references. The only purpose of that table is to record every mention of every law in every other law, but that's unnecessary if Solr's tokenizer is working properly. Eliminate the population of this table from the importer, and replace the method with one that will return the same data, but extract it from Solr instead.