Enhance the HIT preview server

[nalourie-ai2]

Currently, the HIT preview server simply displays the rendered HITs.

The following enhancements could provide a better user experience:

1. Add an index page at the /hits/ URL, linking to all of the HITs.
2. render the HITs in an iframe similarly to how they appear on Mechanical Turk.
3. Add navigation between the HITs (next, previous, and home buttons).
4. Add preview and accept modes to the HITs, similarly to the Mechanical Turk site.
5. Extend the preview server to also render ExternalQuestion HITs.

For implementing 4, it appears that the difference between preview and accept modes for the HITs is the presence of a query parameter, assignmentId=ASSIGNMENT_ID_NOT_AVAILABLE, in the URL used by the iframe [1] [2].