PhET: Add descriptions to sims when available

[nucleogenesis]

Observed behavior

Descriptions

PhET chef does not currently use the "description" field for sims when they are available. They are not available in all languages for every sim, however. For example, open these links and CTRL+F for "description".

English
Spanish
Arabic

Note that the Arabic version of the sim, while available in Arabic does not have a translated description.

This seems to be an unnoticed bug in the download_sim method

Expected behavior

Make the most of the available metadata, give better information to users.

User-facing consequences

Better information about PhET Sims as they explore the Kolibri Library.