Fix sprint ordering for next/prev; fix create sprint form

* next/prev links are calculated based on the sprint start_dates now and
  should correctly link to next/prev sprints
* fixed problems with the create sprint flow where if the sprint slug is
  already used, it would error out, but was throwing an IntegrityError
  and everything was all messed up after that

Fixes #66
Fixes #67
Handle "no bugs" situation better

* fix the various errors that happen when there are no bugs
* display the "no bugs" situation better in the sprint details page

Fixes #68