Allow subtemplates of layout.html to override the LNT page header.

I am going to use this in the global status page to add a wrench icon to
allow for a settings lightbox panel.

git-svn-id: https://llvm.org/svn/llvm-project/lnt/trunk@176927 91177308-0d34-0410-b5e6-96231b3b80d8
Added wrench page header widget image for global status page.

git-svn-id: https://llvm.org/svn/llvm-project/lnt/trunk@176928 91177308-0d34-0410-b5e6-96231b3b80d8
Adding a box shadow/border color effect on the left of the settings icon. Also adding a hover effect to make the settings button discoverable.

git-svn-id: https://llvm.org/svn/llvm-project/lnt/trunk@176929 91177308-0d34-0410-b5e6-96231b3b80d8