Prepare fixtures for Wagtail 1.12

[m1kola]

Because of the reason described in #28 we have to update fixtures to reflect changes from wagtail/wagtail/pull/3285.

Note that we should upgrade wagtaildemo to Wagtail 1.12 first.

[gasman]

@m1kola Good catch! I guess we ought to add a note about this to the "update considerations" section of the 1.12 release notes, for the benefit of any other projects that are using fixtures in this way.

[gasman]

I guess we ought to add a note about this to the "update considerations" section of the 1.12 release notes, for the benefit of any other projects that are using fixtures in this way.

Note added in wagtail/wagtail@8ba95cb.