pull in description property of course pages

[gumaerc]

Pre-Flight checklist

• [ ] Testing
  • [ ] Code is tested
  • [ ] Changes have been manually tested

What are the relevant tickets?

Closes https://github.com/mitodl/ocw-to-hugo/issues/512

What's this PR do?

This PR pulls in the description property of course_pages items and sets it on the front matter of pages. These are plain text descriptions that will be used for meta tags in the head for the purpose of search indexing.

How should this be manually tested?

• Read the readme if you've never used ocw-to-hugo before and make sure you are set up to download courses from S3
• Run node . -i private/input -o private/output -c course_json_examples/example_courses.json --download --rm
• Inspect the courses output to private/output and ensure that the pages under content/pages have a description property, it is plain text and accurate to the page it belongs to

[codecov-commenter]

Codecov Report

Merging #513 (fa5f0d1) into master (75e5b69) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #513   +/-   ##
=======================================
  Coverage   96.33%   96.33%           
=======================================
  Files          18       18           
  Lines        2130     2130           
=======================================
  Hits         2052     2052           
  Misses         78       78           

Impacted Files	Coverage Δ
src/markdown_generators.js	97.22% <ø> (ø)
src/markdown_generators_test.js	90.50% <ø> (ø)

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