Replace ModelAdmin usage with snippets

[Thibaud Colas]

#439 moves the site to the wagtail-modeladmin package. We can go one step further and move to Snippets.

The site’s ModelAdmin usage is all in the one base/wagtail_hooks.py. Excerpt:

class BreadIngredientAdmin(ModelAdmin):
    # These stub classes allow us to put various models into the custom "Wagtail Bakery" menu item
    # rather than under the default Snippets section.
    model = BreadIngredient
    search_fields = ("name",)
    inspect_view_enabled = True


class BreadTypeAdmin(ModelAdmin):
    model = BreadType
    search_fields = ("title",)


class BreadCountryAdmin(ModelAdmin):
    model = Country
    search_fields = ("title",)


class BreadModelAdminGroup(ModelAdminGroup):
    menu_label = "Bread Categories"
    menu_icon = "suitcase"  # change as required
    menu_order = 200  # will put in 3rd place (000 being 1st, 100 2nd)
    items = (BreadIngredientAdmin, BreadTypeAdmin, BreadCountryAdmin)

modeladmin_register(BreadModelAdminGroup)

Our official migration documentation: https://docs.wagtail.org/en/stable/reference/contrib/modeladmin/migrating_to_snippets.html