Blogging Prompts Feature Introduction: use real prompt for new post

[ScoutHarris]

Ref: #18429

When Try it now is selected on the Blogging Prompts feature introduction, a prompt is fetched and passed to the post editor.

To test:

• Enable bloggingPrompts feature flag.
• Select Try it now on the feature introduction.
• Verify the post content matches the prompt. (It should match the dashboard prompt card and FAB header prompt.)
• In BloggingPromptsIntroductionPresenter:fetchPrompt, call completion(nil) in the success block to fake a failure.

bloggingPromptsService.fetchTodaysPrompt(success: { (prompt) in
    completion(nil)
    // completion(prompt)
}, failure: { (error) in
...

• Select Try it now on the feature introduction.
• Verify:
  • The feature introduction is dismissed.
  • An error notice is displayed.

Post Editor	Error Notice

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 pr18597-82f8c5c on your iPhone

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

[wargcm]

Looks good!

[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 pr18597-82f8c5c on your iPhone

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

[ScoutHarris]

Thanks @wargcm !