GFR: Add series to data source for keyword search

[clingerman]

Add series as a data source for the results pulled back in a keyword search.

[DominicBM]

@clingerman: I thought about/looked into this, but I don't think it is logically possible with the way the catalog is set up. Since the record groups don't contain the series-level metadata, we'd have to search all RGs, collections, and series as well, even though we are only doing this to find the RGs/collections with matches in their child series. What this means is every series result would essentially just represent a hit for its RG/collection, but then there could be many hits in the series-level records for the same RG/collection.

So, rather than searching a small set (few hundred records) of unique records as it does currently, we are searching a set of hundreds of thousands of records, and we'd have to scan through the entire set each time to extract the record group numbers/collection IDs for every result and then de-duplicate the list to generate the results on the GFR's end. If you search for a common term like "army", you'll get 58,000+ results at the series level, which makes that workflow basically impossible. Even if we weren't limited by pagination, it would take a very long time to load all of those results to generate the GFR search result set from them.