Reader Comments: Fix crash when the app is backgrounded

[dvdchr]

Refs p5T066-2T7-p2#comment-10881

This fixes a crash due to the table view cells being configured with an attributed string using HTML source while the app is in the background (due to a layout pass that's triggered by the system to get a snapshot).

To test

• Navigate to any comment thread.
• Press Home (or Cmd+Shift+H on the Simulator).
• Tap on the WordPress app again.
• 🔍 Verify that the app does not crash.

Regression Notes

1. Potential unintended areas of impact
   Should be none.

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   Manually tested to ensure the crash isn't occurring.

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]

👍 !