Added UI, route logic for editing categoryAward/Requirement/Weight.

Not really sure how to test this. Also, we should probably put information
about these fields in show-badge.html.

For the most part, though, this fixes #126.
Better server-side form validation error handling.

The server-side form validation is a last resort, but (at least on
badge creation and update) ValidationError responses from mongoose
now return a 422 Unprocessable Entity response with human-readable text,
instead of a 500 with a stack traceback (or nothing, in the case of
production instances).

Coincidentally, this also appears to fix #96, "raw PNG written to console".
Merge pull request #137 from toolness/issue-126-category-points-frontend

Added UI, route logic for editing categoryAward/Requirement/Weight.