Prompts List: don't show table behind empty views

[ScoutHarris]

Ref: #18515
Depends on: #18658

This fixes an issue where the table could be seen behind the loading, error, and no results views.

To test:

• Start with a fresh install to purge any cached prompts.
• Enable bloggingPrompts feature flag.
• Go to the prompts list.
• Verify the loading view is displayed correctly.
• Following the hacks below for each view, verify the error and no results views are displayed correctly.

Hacks:

• To show error view: in BloggingPromptsViewController:fetchPrompts, showErrorView() instead of setting prompts in the success block:

bloggingPromptsService.listPrompts(success: { [weak self] (prompts) in
    self?.isLoading = false
    self?.showErrorView()
    // self?.prompts = prompts.sorted(by: { $0.date.compare($1.date) == .orderedDescending })
}, 
...

• To show no results view: in BloggingPromptsViewController:showNoResultsViewIfNeeded, comment out everything except showNoResultsView():

    func showNoResultsViewIfNeeded() {
//        guard !isLoading else {
//            showLoadingView()
//            return
//        }
//
//        guard prompts.isEmpty else {
//            hideNoResults()
//            return
//        }

        showNoResultsView()
    }

---

Before	After

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Regression Notes

1. Potential unintended areas of impact
   N/A

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 changes in Jetpack from this Pull Request by:

• Clicking here or scanning the QR code below to access App Center
• Then installing the build number pr18662-6ca5379 on your iPhone

If you need access to App Center, please ask a maintainer to add you.

[wpmobilebot]

You can test the changes in WordPress from this Pull Request by:

• Clicking here or scanning the QR code below to access App Center
• Then installing the build number pr18662-6ca5379 on your iPhone

If you need access to App Center, please ask a maintainer to add you.

[wargcm]

LGTM