Add django-treebeard [1] to requirements. Note, that django-treebeard is
already requested by wagtail, because it is also used by wagtail for its
pages.

[1] https://django-treebeard.readthedocs.io/

Update requirements with

    (cd requirements && make)

Use MP_Node for Group and remove priority field.

Add migration. Add the column "path" without the unique constraint
and an invalid value first.

Next, go over all group objects and set a path from a
sequential index. Use alphabetical order, so that menus displayed be
tree are in the same order as before unless sorted by the admin.

As a last step add the unique contraint.

Add TreeAdmin for groups.

Note, that TreeAdmin is based on Djangos ModelAdmin, not on Wagtails
ModelAdmin. So we hook into Djangos admin-interface (available at
/django-admin)

Disable CSRF_COOKIE_HTTPONLY, because TreeAdmin uses Ajax over
http to arrange the group tree.

The <hr> before the "Reset filter" entry is not needed, because its at
the top of the box.

We already have a visual separation between "Reset filter" and the group
names, because the later are in the .dropdown-content div, which as a
little border.

Remove <hr> element.

Add groups in tree order instead of alphabetical order to the
context of the contacts page alphabetical order. Groups will be shown in
that order in the drop-down menu.

Add a method which can be used by a template to get a number of  
entities based on the depth of the group,

Note: Yes, this is an ugly mixture of code and style. Should be fixed up
by some clever CSS using the depth attribute only.

Use Group.get_depth_fill method to get some indention into the group
pulldown menu.