competitions: Change constraints for making a series active

- Remove a constraint that allows only one active series per competition
- Add a constraint that disallows making a series active when it is past
  its due date
problems: Change problem_list.html template tu use base_single.html
problems: Remove ProblemList view from urls and menu
base: Add carousel that displays photo slideshow
competitions: Replace SeasonJoinView with SeasonListView

Rather than displaying a silly form that requires yet another step
before user can see the problems and participate, simply list
all the problems of the active series.
competitions: Add wiring for the SeasonList view