sharing: new method Webdav

I propose to add an option to the sharing module to allow sharing over webdav:

• Apache2 webdav module [1] is already included in FreedomBox
• Linux/Mac/Windows can map webdav directories
• Webdav could cover all Samba use cases https://salsa.debian.org/freedombox-team/plinth/issues/1681#note_118188

Other things to consider:

• May be slower than Samba
• Planned Nextcloud app also uses webdav
• Add Apache2 www-data user to the freedombox-share group

Links: [1] https://httpd.apache.org/docs/current/mod/mod_dav.html