Base sprint progress bars on story points.

Before, the progress bars showed the ratio between
open and closed stories for a sprint. This was fairly
misleading in situations like the folloing:

- Two stories, one is open and one is closed
- The open story is worth 100 points, and the closed
  is worth 1
- The progress bar shows the sprint being 50% done, even
  though there is one huge story left to be done.

Now, the progress bar shows the ratio between claimed
story points and total story points for a sprint. Moreover,
hovering over the progress bar will display the story point
ratio.
Show assignees for each sprint.

Below each sprint's progress bar, there is now
a row of assignee avatars showing who is working
on the sprint.
Allow user to delete stories.

Stories can be deleted by clicking on the "X" in
the top right corner of each story panel.

Also, the "story" label has been removed from the
list of labels in the story modals.