ui: change the order of pending and waiting tasks.

Waiting tasks are closer to being scheduled (they are already given to the
disco_server) so this new order makes more sense.
doc: set the release date for 0.5.2.
event_server: let the task_count be updated incrementally or in the beginning.
job_coordinator: remove a stale comment.
disco_web: do not use max(x, 0) when computing the number of waiting tasks.

This will mask potential issues with computing the number of waiting tasks. We
want to see if the number of waiting tasks is negative to know that there is a
bug in its computation.