Epic: New module: Samba sharing

• Allow to create a public share
• Allow to create a public share for every disk connected to a FreedomBox

Improvements in storage module that would make Samba sharing more error-proof and usable:

• storage: make external disk mounts accessible by other users #1692 (closed)
• storage: run commands before unmounting and after mounting disk #1699
• storage: Create a Django component which makes selecting already shared folder easier for the user #1703 (closed).

Future ideas and discussion points, see also [1]:

• Find common ground with other sharing methods: current web sharing module, MiniDLNA (#1679 (closed), freedombox-todos#65 (closed)), NFS
• Create shared folder on the largest partition of the disk. Some custom Freedombox installations may have large /var or /home partitons
• Full system partition may make Freedombox inaccessible, not enough free space can cause errors when enabling new FreedomBox modules [2]. How should we prevent it from happening?
• Automatically share a folder on mounted removable media, cooperate with storage module.
• Mount removable media with read-write permissions (NTFS and FAT32 filesystems cannot store permissions). To avoid using root user, special unix group (freedombox-share or maybe plinth) should have permissions to access shares. See also #1410 (closed)
• More use cases, private shares.

Links:
[1] https://discuss.freedombox.org/t/rfc-using-external-storage-with-freedombox-applications/228
[2] https://discuss.freedombox.org/t/solved-error-installing-application-unmet-dependencies/463