Unbind task item checkbox events and then rebind them

Fixes #1340

This issue was caused by this line: https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/views/projects/issues/update.js.haml#L17

`new Issue()` gets called on every JS-based issue update, and all the binds in the constructor get re-applied without first unbinding the existing ones. Right now my fix is *only* for the checkbox events, and I suspect there are similar issues still present for other events.

See merge request !478