Allow substructure / similarity search for reagents, products or molecules

We could try to provide substructure search directly on the home page.

For similarity search we could just sort the reaction and product by "best match" even if we can not really suddenly display the similarity value.

No files, tests, or entry points are named. Start by locating the home-page search entry point and the reaction/product result sorting code; done means substructure search is available for reagents, products, or molecules and similarity results can be ordered by best match.

Written by the indexing model from the issue text.