What does this MR do?

Removes the top <p> element added in the field value by MfmService's toHtml. This uses a new inline argument (stolen from toMastoApiHtml).

This fixes Mastodon field link verification.

The top element must be a single <a> in order for verification to work on Mastodon. (as a matter of fact, text-only field values also don't include any elements, so it becomes more consistent in here...)