Change task user to allow multiple assigned users per task

# Problem

GitHub has a limit of 10 assigned users per issue. That means we're going to run into problems if we try to assign more than 10 GitHub users to a task.

In practice, I've never seen that many users assigned, so we could just ignore it as an edge case for now that eventually needs to be worked out.

MVP then would be: can assign multiple task users to a task.

Need to:

- Remove unique index on task and user
- Change task model
- Change task view
- Change preloads