Replace 'result delay' with 'publication date'

Selecting a 'result delay' option is very limiting for admins; this
switches it to a datetime picker allowing them to set an arbitrary time
to post results. On the back end, not much changes: we still track
a result delay by computing the difference in seconds between the end
and publish times.

Example showing form validation:
![pubdate](https://cloud.githubusercontent.com/assets/2406051/2660532/4f343cb0-c021-11e3-97d4-236c1fe98353.png)

Should there be a limit as to how far in the future one can set a result delay?