Strip markdown from og:description meta tags

What does this MR do?

Partially completes #22526 by updating all page_descriptions (which ultimately are rendered as og:description tags) to use the HTML version of the description instead of the markdown version (when available).

This has the effect of stripping all markdown formatting from the text, because page_description strips out all HTML.

Screenshots

	Before	After
Content of og:description tag
Example issues on Facebook
Example issues in Slack