poll: Added postvote and posttrail options for better multi-page polls.

poll: Added postvote and posttrail options for better multi-page polls.

Modern web users probably expect the poll to move on automatically to the
next question, and this allows for that behavior.

Note that bestlink() runs at vote time, which avoids needing to make the
page containing the poll depend on the page that sets up a trail, as the
current trail at vote time will be used.

This commit was sponsored by Eric Drechsel on Patreon.