Comment Detail: Add header cell

[dvdchr]

Refs #17087

This adds the header cell component for the new comment detail. Some notes:

• The header cell is pretty simple, since it's a UITableViewCell with subtitle style (with properly applied text styles for both text label and detail text label). However, since this cell will be reused again later in Comment Threads, I'll create a new class for now to encapsulate all the styling and the .subtitle cell style.
• There are two variants for the header cell content. For this PR, I'll only show the comment's post regardless if the comment is a reply or not.
  • If the current comment is a reply, I'll have to fetch the parent Comment object in order to get the both the replied author's name, and the content. This will be done in a separate PR. I've added a new entry in Comments: Create new comment details view #17087.

Preview:

To test:

• Ensure that the New Comment Detail feature flag is enabled.
• Go to My Site > Comments, and tap on any comment. Preferably not the reply ones.
• Verify that the header cell is displayed.
• Tap the header cell, and verify that the post is opened correctly.

Regression Notes

1. Potential unintended areas of impact
   n/a

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   n/a

3. What automated tests I added (or what prevented me from doing so)
   n/a

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.

[peril-wordpress-mobile]

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

[peril-wordpress-mobile]

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

[ScoutHarris]

Hey @dvdchr . Just a couple comments, but otherwise 👍 !

[ScoutHarris]

As I was testing on iPad, I noticed an odd behavior which I think is related to pushFullscreenViewController. The readerViewController tries to appear full screen, then jumps down to split view. I see this is simply copied behavior from CommentViewController, and I did confirm this is an existing issue in the app store version. But maybe we can fix it with this feature? (at some point, not in this PR)

[dvdchr]

Yeah, that makes sense. I'll note this down in #17087 and address this in a separate PR. Thanks! 🙂