ui: backup: allow to select notification target for jobs

ui: backup: allow to select notification target for jobs

This commit adds a possibility to choose between different options
for notifications for backup jobs:
    - Notify via email, in the same manner as before
    - Notify via an endpoint/group

If 'notify via mail' is selected, a text field where an email address
can be entered is displayed:

    Notify:         | Always notify  v |
    Notify via:     | E-Mail         v |
    Send Mail to:   | foo@example.com  |
    Compression:    | .....          v |

If the other option is selected selected, a combo picker for selecting
a channel is displayed:

    Notify:         | Always notify  v |
    Notify via:     | Endpoint/Group v |
    Target:         | endpoint-foo   v |
    Compression:    | .....          v |

The code has also been adapted to use the newly introduced
'notification-policy' parameter, which replaces the 'mailnotification'
paramter for backup jobs. Some logic which automatically migrates from
'mailnotification' has been added.

Signed-off-by: Lukas Wagner <l.wagner@proxmox.com>