Comment form plugin: fix #198 and enhancement #195 (not for html cleanup)

Fix #198: Pagination/toggle conflict
Enhancement #195:
After an user posts a comment, the page scroll down to the comment posted or to the error message, if any.
The enhancement has to be left open for the html cleanup.

PS
I've added line notes, but I can't see them in the preview. They are in the file commits in my repository though.