Forms/Admin page: support URL parameters for search

[ebruchez]

Following #5274, we'd like to add support for starting a search via URL parameters. Use case: navigating to the list of demo forms from the landing page. At least the app name should be supported, for example:

?fr-search-app=orbeon

We can then also imagine updating the URL when searches are performed, but I'd say that's P2 compared to the first scenario.