Improve Storage Management

[daringer]

• extend nextcloud.Nextcloud() with un/mount() to allow sharing host's mounted devices
• add action for each mounted storage: "mount into Nextcloud"
• define "custom mount command" like: purely custom/raw? presets with defaults: cifs, nfs + raw?
• allow saving "custom mount commands"
• allow selection and execution of "custom mount command"
• add option to allow auto-mounting during nextbox' startup