Only allow searchable layers in query tool

This is to address https://github.com/geoadmin/mf-geoadmin3/issues/2019

Note ( @cedricmoullet  @ltclm ): we have a conflict. `ch.bazl.luftfahrthindernis` is not marked as a searchable layer yet in the db, so with this PR, we can't use the query tool with this layer (haha). So we would need to adapt that. But this would mean that we need to have a sphinx index also for it to be searchable in the search box (as all searchable layers have), but are we ready for this with sphinx? (data is updated on a regular basis).

Alternatively, we could use yet another new flag...but not sure about that, as we already have many quite similar (e.g. selectbyrectangle flag and queryable flag)