Add jinja2 as an extra for accessing formatted html representation of Collections and Items

[pjhartzell]

You only get access to the formatted Collection and Item html provided by _repr_html_ () when jinja2 is installed. This does not appear to be noted anywhere.

I think jinja2 should be included in the setup.py extras_require dictionary and noted in the installation docs in the Dependencies section.

[jsignell]

Given the transition to pyproject.toml there isn't an extra_require anymore but there are specific "project.optional_dependencies". So I guess we could create a new one called display or something? Also this would be closed by #1130 if people like that idea.