web: open graph metadata exposes raw markdown #4661

For example: https://opengraphviewer.com?url=https%3A%2F%2Ftangled.org%2Ftangled.org%2Fcore%2Fpulls%2F4659%2Flatest

The description currently renders as:

Fixes [#4656](https://tangled.org/tangled.org/core/issues/4656) .

Whereas I think it should render the markdown to text first:

Fixes #4656.