Update CI and migrate to Github Actions

[Stormheg]

This PR migrates all our continues integration to Github Actions.

Some of these changes are in the spirit of the Python Package Maintenance Guidelines that Wagtail Nest packages should follow.

• Drop support for Python 3.7
• Update license to include that wagtail-ab-testing is now part of Wagtail Nest
• Remove Travis CI config
• Remove CircleCI config
• Clean up tox config and fix some minor issues with running tests
• Move nightly test against Wagtail's main branch to Github Actions
• Move Javascript formatting checks to Github Actions
• Move Python checks to Github Actions
• Update readme badges
• Make sure the readme renders on PyPI.org
• Add section on where to report security issues

[Stormheg]

Thanks @SaptakS!