Fixed various issues with the homepage and elsewhere on the Isis website. The...

Fixed various issues with the homepage and elsewhere on the Isis website. The navigation menu has been removed because it no longer serves a purpose after the webteam redesigned the Astrogeology website. I fixed several broken links on the homepage and the Isis download page. The homepage no longer has the 'Whats up at the Isis support center' section as this was not properly grabbing the latest posts from the support board. The 'Announcements' section points users to the support board for the latest announcements, which was what it was already doing when the website was not 'live'. Fixes #666.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@3883 41f8697f-d340-4b68-9986-7bafba869bb8