fix #5093: ui: acme: expose custom directory option

fix #5093: ui: acme: expose custom directory option

This patch allows the user to set a custom ACME directory by providing
a 'Custom' option in the directory dropdown. This in turn reveals an
input for the url. When using a custom directory the directory has to
be manually queried via button press to prevent from spamming the
directory on every input.

Signed-off-by: Folke Gleumes <f.gleumes@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>