This big commit adds a separate module hierarchy for Servant named
routes (see https://www.tweag.io/blog/2022-02-24-named-routes/ ) which
will make working with servant endpoints more pleasant (especially when
it comes to emitted errors).

This still doesn't do anything to wire the routes to the concrete
handlers.

This commit removes the need for a custom Opaleye fork by relying on
a newly-created `opaleye-textsearch` package following the design
outlined here:

https://github.com/tomjaguarpaw/haskell-opaleye/pull/563#issuecomment-1782794439

In my previous commit I had changed their implementation to use the
FromJSON/ToJSON istances, making sure those were backward compatible,
but apparently that wasn't the case.

After Fabien's changes, this needed amendment.