Replace built-in file manager

[Raruto]

Checklist

• [X] I've searched through the current issues to make sure this feature hasn't been requested already.

Motivation

Current file manager app looks a bit outdated (eg. currently not configured with "drag-and-drop" support) and "no longer supported".

Suggested solution

Replace RichFilemanager with Filestash

Alternatives considered

Remove the file manager as a core feature and install "Filestash" exclusively as a docker service?