Return leading image for {{content words="0"}}

Closes #3884
- The custom parsing first checks for a leading markdown image. If none
is found, it will check for a leading <img> tag (or a <img>  inside a
leading <p>).
- A leading Markdown image tag will be parsed and the resulting HTML
returned. A leading HTML image tag will be extracted and returned.
- Before/After-Behavior is identical.
- The tests have been updated to also submit markdown and to test img
tags the way they are currently being returned by Showdown.