Wagtail version 3+ compatibility

The package is currently limited to wagtail version >1.4 and <3.0. All of those versions are out of support: https://github.com/wagtail/wagtail/wiki/Release-schedule.

I propose to submit a PR that will:

• fix the panels in the MetadataPageMixin so it will continue to work with Wagtail versions 3+;
• increase minimum requirement for Wagtail to version 2.13, because it's the minimum version that supports Django 3.2;
• upgrade the example app to use Django 3.2 and Wagtail 4.1, because those are LTS releases;

I think we could also drop support for Python 3.6, and add support for more recent Python versions.

I propose deploying the changes as a new version 3.0, because it will have breaking changes.

please do

On second thoughts, trying to maintain compatibility with wagtail versions before 3.0 looks too hard. The trouble is the changes to module paths (https://docs.wagtail.org/en/stable/releases/3.0.html#changes-to-module-paths) in Wagtail 3.0.

I started amending wagtailmetadata/models.py like this:

from wagtail import VERSION
if VERSION[0] >= 3:
    from wagtail.admin.panels import FieldPanel as SearchImagePanel
    from wagtail.models import Site
else:
    from wagtail.core.models import Site
    from wagtail.images.edit_handlers import ImageChooserPanel as SearchImagePanel

And then using panels = [SearchImagePanel("search_image")] in the MetadataPageMixin. That looks OK by itself to me.

However, there are many more locations where changes like that are needed. For example, in wagtailmetadata/tests/settings.py I used:

if VERSION[0] >= 3:
    wagtail_app_str = "wagtail"
else:
    wagtail_app_str = "wagtail.core"

so that I could use that wagtail_app_str in the INSTALLED_APPS and MIDDLEWARE settings.

But then I encountered the example/puput_migrations directory, and amending that for dual versions starts to become too impractical.

So I now propose to increase the minimum Wagtail version to 3.0. Users with older wagtail version will need to continue using a version 2 of this package.