Notifications > Comments: fix notification selection issues

[ScoutHarris]

Ref: #17790

This fixes two issues when selecting a Comment notification:

1. On a fresh install, the app would do nothing and become unresponsive. This was due to createViewController:comment sometimes returning before fetchParentCommentIfNeeded was complete. This resulted in CommentDetailViewController being nil.

To resolve this, a completion block has been added to createViewController:comment so it doesn't return until the view controller is initialized.

2. Comment details would be from the previously selected comment notification.

This has been resolved by clearing the previous notification's properties when a new notification is displayed.

To test:

• Enable notificationCommentDetails.
• Start with a fresh install to ensure no Comments are cached.
• Go to Notifications > Comments.
• Select a notification.
• Verify the comment details view is displayed (and the app doesn't hang).
• Go back to the Notifications list and select other comment notifications.
• Verify the comment details are for the notification selected.

Regression Notes

1. Potential unintended areas of impact
   N/A. Feature is disabled.

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.

[wpmobilebot]

You can test the Jetpack changes on this Pull Request by downloading it from AppCenter here with build number: pr17978-e621aa2. IPA is available here. If you need access to this, you can ask a maintainer to add you.

[wpmobilebot]

You can test the WordPress changes on this Pull Request by downloading it from AppCenter here with build number: pr17978-e621aa2. IPA is available here. If you need access to this, you can ask a maintainer to add you.

[dvdchr]

Works as expected, LGTM!

Left some optional stylistic suggestions but I'll leave it up to your preference. 🙁

[ScoutHarris]

Thanks @dvdchr !