Bump wagtail from 2.7.1 to 2.8

Bumps [wagtail](https://github.com/wagtail/wagtail) from 2.7.1 to 2.8.
- [Release notes](https://github.com/wagtail/wagtail/releases)
- [Changelog](https://github.com/wagtail/wagtail/blob/master/CHANGELOG.txt)
- [Commits](https://github.com/wagtail/wagtail/compare/v2.7.1...v2.8

)

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