Add group template to settings

Hi, and thanks for the plugin!

Although it has its limitations - what plugin doesn't have - it is fairly usable.
The biggest issue for me (besides everything related to placeholders vs API deficiencies as described in https://github.com/nextcloud/ldap_write_support/issues/122) is group creation.

I use this schema for Nextcloud's user and group objects. Furthermore I add uidObject objectClass to all groups, so they have uid field, which is used in their dn. This makes the following Nextcloud LDAP configuration possible:

• I filter LDAP groups in Nextcloud based on objectClass=NextcloudGroup
• I set groups' displayName to cn, which possibly contain non-ASCII chars (as I manage a Nextcloud instance for Hungarian members)
• I set groups' UUID to uid which is all lowercase ASCII (and practically the English equivalent of displayName-giving cn)

Now, group creation with this setup - obviously - fails.
Groups created through Nextcloud are added to LDAP, but they are not found and parsed back (Nextcloud reports group creation failed) because:

• they don't get the NextcloudGroupobjectclass
• they don't get the uidObject objectclass, so they cannot get the uid attribute, even if creation would take my LDAP group UUID settings into account.
  This could be mostly fixed with a similar template editor as given for user creation.
  I'd add these two objectClasses to the template, set uid and cn to groupName supplied by Nextcloud. I'd still have to change cn in LDAP later, but the non-technical board members of the NPO (for which this Nextcloud is set up) could create the groups without me, and I could "re-displayName" them when I have the time...

Review open pull request #219 first, then compare the requested group support with the existing user-creation template editor mentioned in the issue. Confirm that group creation can supply the requested objectClasses and groupName-derived uid and cn values, and verify the resulting groups are found and parsed by Nextcloud.