Fix error message generation for site notices.

- Prevent 'Cancel' link from generating duplicate error lightbox.
Refine css for  tabs in class materials page.

- Refine css for  tabs in class's materials page.
Merge UI fixes

- Merge branch 'ui_fixes_for_cc_02_20_2013_a' into
  add_devise_from_scratch_02_20_2013_a