Sandbox

Fixed [Issue 78845090] (https://www.pivotaltracker.com/story/show/78845090) (Show HTML button not working)
  - extracted `h(@code.tpl)` to helper method
  - created directory test/unit/helpers to house helper tests and added file application_helper_test.rb to house tests of application_helper
  - added test of above helper method
  - fixed helper so that test passes

Trivial to minor refactoring of sandbox view, including
  - change Sandbox output border color (partly white border was partly invisible in BlackOnWhite color scheme)
  - move most styling into stylesheet