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].