sharing: Simplify creating public, group and home web shares

I propose to show the same kind of share selection table on the sharing page as is on the samba page. Then admins can easily enable group and home shares on particular disk for the users who are in the freedombox-share group.

If a home share is enabled, users can create a public share by creating public_html subdirectory under the home share (which is similar to the Apache2 UserDir feature), public share URL will be then https://hostname/share/diskname_home/username/public_html/ .

Antoher option I'm thinking about - instead of modifying the sharing app, add a button to the samba page that enables accessing samba shares over HTTP.

Unlike samba shares, web shares are read-only and accessible also from external networks.

Related to #1755