Doesn't save "top-level" resources

[JohnVeness]

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Subject of the issue

There are a number of downloadable resources mentioned at the top-level of courses, shown in a sidebar on the right, that aren't currently being downloaded by edx-dl.

Your environment

• Operating System (name/version): Ubuntu 20.04
• Python version: 3.8.2
• youtube-dl version: 2020.03.24
• edx-dl version: 0.1.13

Steps to reproduce

1. Visit https://courses.edx.org/courses/course-v1:MITx+6.002.1x+2T2019/course/ in a web-browser
2. Observe the "Course Handouts" list on the right
3. Download course with edx-dl

Expected behaviour

All the Course Handouts should be downloaded.

Actual behaviour

Many of the Course Handouts are not downloaded (the only ones that are, are ones that are also linked from individual units).