Post Details Comments: add table header with comments count

[ScoutHarris]

Ref: #17511

This adds a custom table header view for the Comments table that displays the total comment count.

I also made a couple changes to the BorderedButtonTableViewCell:

• Fixed an issue where overriding the button insets didn't actually work. 🤦
• Set the default button insets to 0.

⚠️ Auto-merge enabled ⚠️

To test:

• Enable the postDetailsComments feature.
• Go to Reader > post with comments > details.
• Verify the XX Comments header is displayed with the total number of comments.

Regression Notes

1. Potential unintended areas of impact
   N/A. WIP.

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

3. What automated tests I added (or what prevented me from doing so)
   N/A. WIP.

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.

[ScoutHarris]

Hey @dvdchr .

If changes are needed that prevent approval, would you mind removing the 18.8 Milestone? Thank you!!

[peril-wordpress-mobile]

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

[dvdchr]

Found a few visual issues, and nothing major. Approving since this is still WIP! 👍

• Somehow the header view is displayed with a gray background for me, but it is displayed correctly in your screenshot 🤔 .
• I just realized while checking the designs in Zeplin earlier, but I think the "View All Comments" button should have a lighter gray border instead of having the .text color... (maybe it should be separator?). Here's the design from Zeplin:
  

[ScoutHarris]

Hey @dvdchr .

the header view is displayed with a gray background for me

Huh, that's not good. I'll take a look! Thanks!

button should have a lighter gray border

Yea, I noted that when I added the button: The design has the button border and label different colors. I'll add this functionality in a follow-up PR.

I just haven't done it yet.

[dvdchr]

Yea, I noted that when I added the button:

Oh, got it! 👍 Sorry, that must've escaped me.