Make Website URLs relative to document root

Removing reference to mantisbt.org allows local testing
Remove redundant 'content-type' meta tag
Fix issue when builds dir is a symlink
Support page: fix 'Docs' button's vertical alignment
Replace IRC channel #mantisbt-help by #mantisbt
Support page: use h1 header for IRC channel
Move code to display error messages to helper function
'Voor' sounds like Dutch
More Dutch stuff...
Fix system notice
Remove further references to #mantisbt-help IRC channel

It appears that the irc.php page is not referenced anywhere on the new
website, so this update may not be necessary; nevertheless for
consistency it is best to remove all references to the old help channel.