Notification Comment details: show loading and error views

[ScoutHarris]

Ref: #17790

When a comment is being loaded, a loading view is displayed. When loading a comment fails, an error view is displayed.

To test:

• Start with a fresh install so no comments are cached.
• Slow down your network connection.
• Go to Notifications.
• Select a Comment notification.
• Verify:
  • The loading view appears.
  • The loading view is removed when the comment is loaded.
  • NOTE: the gap at the top of the view as noted on this PR is still present. This will be addressed later.
• Navigate to other notifications via the buttons on the nav bar.
• Verify:
  • The loading view appears.
  • The loading view is removed when the comment is loaded.
• Implement the hack below to show the error view.
• Select a Comment notification.
• Verify the error view appears.

Hack to test the error view. In NotificationCommentDetailViewController:

• Immediately return nil from loadCommentFromCache.
• fetchComment - call completion(nil) in the success block instead of completion(comment).

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.

[wpmobilebot]

You can test the Jetpack changes on this Pull Request by downloading it from AppCenter here with build number: pr18062-50f817b. 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: pr18062-50f817b. IPA is available here. If you need access to this, you can ask a maintainer to add you.

[dvdchr]

Things are working as described. LGTM!

I've found a regression that's unrelated to this PR, but thought to share here. So when you get a comment notification from a site that you don't have access to, the Notification Comment Detail shows an error view. In the previous Notification Comment Detail, it worked.

Here's the repro step:

1. With a second account, create a comment on a publicly accessible WP.com site (preferably owned by your other account).
2. Now switch to your primary account, and reply to your second account's comment. This should trigger a reply notification on the second account.
3. Switch back to your second account in your WordPress app, and go to Notifications.
4. Tap on the reply notification.
5. Observe that the page displays an error.

I've taken a look, and this is most likely because loadComment expects a Blog instance from local storage – and since the second account doesn't have any roles, the app didn't fetch the site, causing the blog to be nil.

Maybe we can switch the parameter to blogID instead, and use restRemoteForSite:(NSNumber *)siteID instead of remoteForBlog:(Blog *)blog?