Wagtail 3.0 compatibility

[nickmoreton]

Add support for Wagtail 3.0

This PR does mean the package now requires Wagtail 2.15+ I noticed some issues with the latest Jinja2 version so have pinned that and markupsafe

jinja2>=2.10,<3.0,
markupsafe==2.0.1,

Tasks

• [x] Update docs
• [x] Update Change Log
• [x] Change import paths
• [x] Update tests
• [x] Update tox config

[nickmoreton]

Hi @seb-b It would be great if you could review this for us please and get anew release out. Thank you.

[seb-b]

Thanks @nickmoreton! Just released version 3.0.0 with this pr in it