Consider filestash as filemanager

[rgaudin]

https://github.com/mickael-kerjean/filestash has a clean, minimal UI that's more appropriate than filemanager for our use case. It also supports the filetypes we need.

The main difference is that it is a filemanager UI that relies on a external backend. We would thus use it via WebDAV, separating the file API and UI which allows more use cases.