Show acceptable formats tooltip for input-number and expression

Summary:
Adds a tooltip to input-number, numeric-input, and expression,
which shows the acceptable formats for that widget when they
are not located in the answer area.

Test Plan:
* Add an expression and an input number
* See they both have the tooltip that show up on focus.

Reviewers: justinjaffray, eater, alex

Reviewed By: alex

Differential Revision: http://phabricator.khanacademy.org/D8511