Reader Comments: Refactor content cell to have configurable render method

[dvdchr]

Refs #17476

This PR refactors CommentContentTableViewCell so it could render its content in several different ways.

• The web view in the cell is now replaced with an empty UIView. Depending on the rendering method, a WKWebView might be added to the UIView – or it could be a UITextView later on.
• No new method has been added just yet, so this should still render the web view as before.
• As this is a non-user facing change, please ensure that things look the same as before.

To test

Site Comments

• Go to My Site > Comments.
• Select any comment.
• 🔍 Verify that the comment cell is displayed correctly and is not impacted by this change.

Comment Threads

• Ensure that the newCommentThread is enabled.
• Go to Reader > any comment threads.
• 🔍 Verify that the comment threads are rendered with the web view method (same as before), and not impacted by this change.

Regression Notes

1. Potential unintended areas of impact
   n/a. This feature is still hidden behind a flag.

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   n/a. This feature is still hidden behind a flag.

3. What automated tests I added (or what prevented me from doing so)
   n/a. This feature is still hidden behind a flag.

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

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You can trigger an installable build for these changes by visiting CircleCI here.

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You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[ScoutHarris]