Adjust app to NC 20 and unified search

[raimund-schluessler]

With the unified search on NC 20, the old search box is not available anymore. We have to realize filtering differently now.

Also at the moment you get a warning that

[Vue warn]: Error in mounted hook: "Error: searchBox not available"

(found in <Root>) vue.runtime.esm.js:619

but since this warning is issued in https://github.com/nextcloud/server/blob/master/core/search/js/search.js#L62 and you get it in every app using the old search I guess this is a server issue? @skjnldsv

[skjnldsv]

This is fixed already with nextcloud/server#22099
We now warn the deprecation :)

[raimund-schluessler]

Needs clarification from server as how to proceed with content filtering: nextcloud/server#22092 (comment)