Add Kolibri HTML5 Article constants.

[rtibbles]

Summary

• Adds constants for a new Kolibri HTML5 Article type (a zip format that is almost identical to our HTML5 App, except it is intended to be displayed sanitized and without any injected interactivity).

References

In support of learningequality/kolibri#13274

Reviewer guidance

Note I deliberately did not add these to the file_types.py module which seem to be only used for automatic mapping in the ricecooker JsonTreeChef workflow.