Do not count disabled tasks for challenge metrics

[mvexel]

When a challenge has disabled tasks, they are currently counted for the total number of tasks available in the challenge. This is unexpected behavior because the disabled status is not something the end user sees. Tasks that are disabled do not show up when mappers are working on a challenge, and they do not show up as a status in the challenge page.

Proposed Fix: Do not count disabled tasks when computing the counts in the challenge overview screen.

See maproulette/maproulette-backend#1092 (comment) for background.