Embed external Doenet activities

[cqnykamp]

This PR adds the syntax \doenet{...embed code...} to Ximera.

Parameters:

1. embed code - this is a UUID found from the Doenet beta.doenet.org website. Note that this ID is not a hash! The content of the embedded Doenet activity could change without the ID changing.
2. width
3. height

Limitations

• The iframe around the Doenet activity does not expand to match the changing height
• Doenet's answer system doesn't integrate with Ximera's answer system

Examples

See this demo repository for some examples of Doenet in Ximera.