Sync comments content only if needed

[ealeksandrov]

Fixes #17202.

Description

On each viewWillAppear method syncContent is called to reload all data, even if there is something already loaded.

The fix is adding a check and call syncContent only when it's needed (no loaded content).

To test:

1. Run the app on an any device/simulator.
2. Select a site that has many comments, such that it has two or more pages when scrolling down the comments list.
3. Tap My Site > Comments.
4. Scroll down until the fetch request for more content is triggered.
5. Select any comment on page two.
6. Tap Back.
7. Check that the Comments list is still at same page and scroll position doesn't jump.

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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.

[dvdchr]

LGTM!

Tested:

• Scrolled to page two or more on the comments list, and tapped on one Comment. When going back to the list, the paging should be preserved.

Additional test scenarios:

• Comments list content snippet should reflect the updated comment data.
• Comments list should correctly remove any comment that's marked as spam or trash from the list.

[ealeksandrov]

Thanks for revew David!