Fix exception in Maniphest task ID filtering

Summary:
Ref T2632. When the user enters task IDs, we filter them to allow the user to write `T123` or `task 123` to mean `123`. This filtering is latin-centric and silly, and cuases an exception when accessing, e.g,, `/maniphest/view/custom/?tasks=~`.

Instead of stripping a select few nondigits, strip all nondigits.

Test Plan: Hit `/maniphest/view/custom/?tasks=~`, no exception.

Reviewers: AnhNhan, chad, vrana

Reviewed By: chad

CC: aran

Maniphest Tasks: T2632

Differential Revision: https://secure.phabricator.com/D5193