fix google classroom share link

[stephiescastle]

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Description

Supports https://github.com/nasa-jpl/www/issues/241

• adds the necessary url prefix for google classroom
• modifies mixinCanonicalUrl to gracefully handle instances when the path includes the domain (noticed that locally that the url field for pages was returning fully qualified urls, whereas on production, the url field returns a path).

Instructions to test

Tested in the following environments/browsers:

Operating System

• macOS
• iOS
• iPadOS
• Windows

Browser

• Chrome
• Firefox ESR
• Firefox
• Safari
• Edge