Added mobile support to numeric-input and removed interceptInputFocus

Summary:
Refactored InputNumber, moving all the static rendering logic down to InputWithExamples so
that it can be shared by all three input components that perform such an operation, namely NumericInput
and Expression. As a result, NumericInput also works on mobile (with minimal additional work).

In the process, I was also able to remove the interceptInputFocus code. Given that we're now stubbing in
a static rendered input, I don't think the iOS app will need to intercept focus any more.

Test Plan:
- Create a page with a bunch of inputs.
- Click around and make sure only the clicked input has the "perseus-focused" class.
- Use the console to itemRenderer.focusPath(itemRenderer.getInputPaths()[i]) for some values of i. Verify that only one input has "perseus-focused".
- You may also want to run the iOS app with this version of Perseus and verify that inputs work as expected, including arrows, etc., given that this is built on that commit.

Reviewers: jack

Reviewed By: jack

Subscribers: mgp

Differential Revision: http://phabricator.khanacademy.org/D11471