Minor fixes related to line breaks & Markdown in metadata fields

[ybnd]

References

• Add support for line breaks markdown and mathjax in metadata #1851

Description

This PR bundles a few minor fixes related to #1851

• Even spacing between comma-separated MDVs (e.g. subjects)
  

  This was an older issue, caused by line breaks within the <span>, which Angular translates into unwanted spacing. Now we include a space in the separator itself: ', '

• Made enableMarkdown an input of ds-generic-item-page-field so it's easier to enable Markdown for arbitrary Item page fields

• When the new dsMarkdown pipe is combined with the new .preserve-line-breaks CSS class, line breaks in Markdown are effectively doubled. Therefore, we shouldn't add this class when rendering Markdown.

Instructions for Reviewers

Confirm that this PR solves the issues listed above and that the changes are sensible.

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[ybnd]

The e2e failures seem entirely unrelated to the changes here, so I'm hoping they're random.

[tdonohue]

👍 Thanks @ybnd ! I gave this a test today and markdown still works everywhere, and it looks better (with smaller line breaks). Code looks good too!