Fix utf8 issues in calls to md5_hex.

Fix utf8 issues in calls to md5_hex.

This prevented comments containing some utf-8, including euro sign, from
being submitted. Since md5_hex is a C implementation, the string has to be
converted from perl's internal encoding to utf-8 when it is called. Some
utf-8 happened to work before, apparently by accident.

Note that this will change the checksums returned.

unique_comment_location is only used when posting comments, so the checksum
does not need to be stable there.

I only changed page_to_id for completeness; it is passed a comment page
name, and they can currently never contain utf-8.

In teximg, the bug could perhaps be triggered if the tex source contained
utf-8. If that happens, the checksum will change, and some extra work might
be performed on upgrade to rebuild the image.