Bump wagtail from 2.4 to 2.5.1
Bump wagtail from 2.4 to 2.5.1

Bumps [wagtail](https://github.com/wagtail/wagtail) from 2.4 to 2.5.1.
- [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/v2.4...v2.5.1)
marteinn/wagtail-alt-generator,marteinn/wagtail-alt-generator,marteinn/wagtail-alt-generator