Add featured image support

[adamwoodnz]

Add Featured Image support, for display in homepage cards on learn.wordpress.org, see WordPress/Learn#2394

[renintw]

Some updates from previous findings:

Initially, I spent some time on meeting-calendar, but due to a bunch of outdated/deprecated packages, I was unable to build it locally, and it also caused the GitHub action to fail continuously. I tried to fix the errors as they appeared, but it only resulted in more errors. Earlier I attempted different methods, but they were still ineffective. A comprehensive review and upgrade of the packages are necessary. I opened an issue and recorded some notes in it.

Additionally, I confirmed that the source and retrieval method for the feature image of the online workshop is not through the meeting calendar. The meeting calendar actually merely takes data from an attribute and renders it into the calendar (ref). The actual data is obtained through meetup.com. However, the data from meetup.com is not directly retrieved via its API. Instead, it is stored in a database from somewhere and then fetched by wporg-learn (ref). And the image URL data we need is not stored in the database. So we need to identify which part in the code the meetup.com data gets stored in the database and then modify that part to retrieve the image URL from the meetup.com API and store it in the database.

According to the official docs, the event object appears to include image information.

[adamwoodnz]

Let's deprioritize this in favor of the Course and Lesson issues. We can come back to it during QA.