Stats Insights: strip html from post title.

[ScoutHarris]

Fixes #17023

This strips HTML tags from the post title when displayed in the Latest Post Summary block.

To note, I poked around other post titles in Stats, and we do strip HTML elsewhere, so I did it here to match.

To test:

• Create a post with HTML in the title (like <em>My awesome title</em>).
  • You may want to do it in a browser. When created in the app, the HTML is treated as regular text. It is stripped correctly in Stats, it just looks weird elsewhere.
• Go to Stats > Insights > Latest Post Summary.
  • If it's not listed, you may need to add it via the Add stats card option at the bottom.
• Verify the HTML is not displayed in the post title.

Before	After

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.

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You can trigger an installable build for these changes by visiting CircleCI here.

[ScoutHarris]

Hey @dvdchr . Would you mind another random review please? Thank you! 🙇

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[dvdchr]

LGTM!