Feature: include comments in advanced search

[shamoon]

Proposed change

This was on the list back when comments were added, hopefully should make them more useful. Adding them to the search was pretty straightforward (first commit), I danced with whoosh a bit for the permissions stuff. But adding the highlighting Im not so totally certain about, open to other solutions, but this seems to work.

Fixes #1654
Fixes #687
See #1375

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Other (please explain)

Checklist:

• I have read & agree with the contributing guidelines.
• If applicable, I have tested my code for new features & regressions on both mobile & desktop devices, using the latest version of major browsers.
• If applicable, I have checked that all tests pass, see documentation.
• I have run all pre-commit hooks, see documentation.
• I have made corresponding changes to the documentation as needed.
• I have checked my modifications for any breaking changes.

[github-actions]

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