Change BlurInput to NumberInput

Summary: Because I like it better. And because I'm going to "backport" changes from ValidatedNumberInput to NumberInput at some point, including things like labels, validation, key bindings, css improvements and whatnot.

Test Plan: Shouldn't change interpretation, but should work onChange rather than onBlur

Reviewers: eater

Reviewed By: eater

Subscribers: alpert

Differential Revision: http://phabricator.khanacademy.org/D7358