Unhide legacycustomsearches, with a view to moving away from install-by-default

We moved most of the legacycustomsearch code to an extension around a year ago. The extension is hidden at the moment but we should unhide it to make it easier to uninstall (and eventually to stop installing on new installs)

The main issue with uninstalling it is that some extensions depend on it but do not list it as a dependency - I will add a note in the dev list about that. I think that is no longer blocking on unhide

I think there are still a couple of bits of code to move to the extension in core