auth ui: add LDAP realm edit panel

auth ui: add LDAP realm edit panel

The panel was mostly taken from from PVE, but altered slightly:
  - bind-dn and bind-password are displayed under "General"
    and not under "Sync". For some servers, we need to be bound
    to lookup a user's domain from a given user id attribute.
    In PVE, the bind-dn and bind-password fields are under
    "Sync", which is a bit confusing if a user is not interested
    in automatic user syncing.

  - There is a 'anonymous search' checkbox. The value is not persisted
    in the configuration, it merely enables/disables the
    bind-dn and bind-password fiels to make their intent a bit more
    clear.

  - Instead of a 'secure' checkbox, a combobox for TLS mode is shown.
    This way users can select between LDAP, STARTLS and LDAPS.
    In PVE, the 'secure' config parameter is deprecated anyway, so
    I took the opportunity to replace it with the 'mode' parameter
    as described.

  - Parameters now consistently use kebab-case for naming. If
    PVE is modified to use the same panel, some sort of adapter
    will be needed.

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