Add exact string search

[hexhowells]

Currently the search utilises vector search with encoding transformers for feature vectors. But even with the exact paper title in the search query; sometimes the paper doesn't show up in the results.

Should implement exact string search using double or single quotes around the part of the query to match.

May need to combine this with the current search to also show results after the exact string search results are shown. Also need to be able to properly process the search query when there is a mixture of quoted and non-quoted strings in the query.