Use more explicit language for unassigning tasks

Summary:
Ref T10493.

• Call this action "Unassigned" instead of "Up For Grabs", since the latter implies that it's OK for anyone to grab it, which is a social/context thing that we probably shouldn't assume.
• Show who a task was previously assigned to in the transaction.

The text is a little clunky, yell if you've got a better wording? Or maybe I'll come up with something.

Test Plan:

Reviewers: chad

Reviewed By: chad

Subscribers: cburroughs

Maniphest Tasks: T10493

Differential Revision: https://secure.phabricator.com/D15454