form: add Proxmox.form.field.ExpireDate

form: add Proxmox.form.field.ExpireDate

Behaves like a 'datefield' but treats the 0/empty value as "never",
and sets all defaults for behaving like one would expect from a date
expire field.

This allows to replaces hacks (e.g. [0]) from our various expire date
fields, mostly in PVE/PMG user accounts, and also for the soon to be
applied API token gui.

[0]: https://git.proxmox.com/?p=pve-manager.git;a=blob;f=www/manager6/dc/UserEdit.js;h=40c4044fd9364c9bcb8787507068378ed9936f67;hb=806edfe19f049f07db2628c68e4e5702bbedd9d7#l159

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>