feat: split struct org data from SeoMixin

[Chadys]

Fix #60 An alternative solution from #63 to remove struct data from SeoMixin, by creating SeoOrgMixin instead.

I find this solution much less risky, the impact for the upgrade is lessened compared to the settings approach, there was no need to change tests or templates and no manual code is required for the migrations.

[vsalvino]

Amazing, thank you. I will review these two hopefully at the Wagtail Space US sprints :)

[vsalvino]

Merging this in via #71