ui: firewall: refactor privilege checks and prevent double click

ui: firewall: refactor privilege checks and prevent double click

factor out the relevant privilege checks in a variable and reuse that,
also add the check in the run_editor (or wrap it with a check) so that
the edit windows don't open with a double click without those privileges

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>