Wagtail-draftail-snippet

Upgrade required version of Wagtail to >=2.15.x (feature implemented in 0.4.0)

Support Django 4.x and Wagtail 3.x

Fix issue when upgrading to Wagtail 2.14.x.

Fix issue when upgrading to Wagtail 2.11.x.

Adds the ability for snippets to be either a link (current functionality) or a block element.

Breaking changes

- `snippet` Wagtail feature renamed to `snippet-link`
- template search looks for `{model}_snippet_link.html` instead of `{model}_snippet.html`

Update travis-ci configuration.