[www] Fix links to bug tracker

The LLVM bug tracker is now available at bugs.llvm.org instead of llvm.org/bugs.
By updating our links to the tracker we do not only avoid unnecessary redirects,
but also certificate warnings.

We use this opportunity to shorten the text and to rename the link 'open bugs'
to 'show open bugs' to clearify its meaning.

llvm-svn: 304768