Wagtailgridder

What's Changed
* Support for Wagtail 3 and above. By rgs258 in https://github.com/wharton/wagtailgridder/pull/15
* Moved to SemVer and stable release. By FlipperPA

**Full Changelog**: https://github.com/wharton/wagtailgridder/compare/0.14.0...1.0.0

What's Changed
* Added Django 4.0 text (`ugettext` -> `gettext`)
* Fix the default caching period of one week.
* Use Python 3 types instead of names.

**Full Changelog**: https://github.com/wharton/wagtailgridder/compare/0.13.2...0.14.0

* This targets the cache clear to only occur when a WagtailGridder page type is edited, rather than any Wagtail page.

* Bug fix: fixes an issue with module resolution order.

* Creates an abstract model that can be customized by the developer.

* To maintain the previous behavior, set `WAGTAILGRIDDER_GRID_INDEX_PAGE_SUBPAGE_TYPES = None`
* `GridItem` pages can still be included on multiple `GridIndex` pages, this just enforces some nice housekeeping by default.

Thank you to Ryan Sullivan for the Pull Request!