First pass at hint request experiment.

Based on a value returned by the server, hints are either shown as normal or
users have to press a button to see the hint.  The placement of the "request hint"
button relative to "try again" is also controlled by the value returned by the
server.  If no value is returned, the experiment does not take place
(hints are shown as usual).

Dashboard side at: https://github.com/code-dot-org/dashboard/pull/1047
Wondering if there might be a way to DRY this template up, but can't think of a good one off the top of my head short of pulling out partial templates for each button, which seems excessive, and even then you might then need to pass through the available variables. Ick.

If there was a clean way to define partials in-line that could work (e.g., `<% var tryAgainbutton = %> {some erb} ...`), but I'm not sure there is.