R1:99bc1b05d701

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: {F1166299}

Reviewers: chad

Reviewed By: chad

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