www: link to the planet clang newsfeed

Also shuffle the Communication section so the bug tracker comes first.

(The sidebar isn't scrollable at the moment so this gives a better chance of
the bug tracker being seen. The links further down are basically invisible --
we should look into that.)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199398 91177308-0d34-0410-b5e6-96231b3b80d8