Adding summary of recent Disqus comments to home page.

This closes #528.

Bug: The excerpts have `<p>` tags displayed as text.
Merge pull request #712 from gvwilson/adding-disqus-to-home-page

Adding summary of recent Disqus comments to home page.
Add link to iPlant workshop at U of Arizona
iPlant workshop
Cleaning up nav bar
Merge branch 'master' into adding-scf-material