This commit mimics 100% the behavior achieved with the
parent commit of this when using the advance option.

1) Rename the 'tohtml' option to 'topreferredformat' to better
represent what it's doing.

2) Always honor the preferred format (this is a new post).

3) Only perform the conversion to HTML when the preferred format is
HTML.

That way any user, no matter thy are using format MOODLE (plain textbox),
MARKDOWN (marklar editor) or HTML (atto and tinymce editor) will
get always the format correct and the contents ready to be edited if
desired. HTML ones converted to HTML and TEXT ones unmodified.