Fix input-overwriting on table widget editor

Summary: Cleaned up some of the inputs in the table-of-values widget editor so that you can more easily enter numbers (fixes T3021) and fixed blur/focus errors on the inputs (because of missing onFocus/onBlur default functions). I also removed a very lonely radio button ("Set of values (complete)") but can add it back if it's actually necessary.

Test Plan:
- Add a table widget
- Increase/decrease the number of columns and rows
- Observe that everything behaves as it did before (including grading) except you don't throw your computer out the window trying to enter a number other than 1 or 6 for the number of columns
- Observe that the # of rows are still constrained between 1-30, and the columns between 1-6

Reviewers: aria

Reviewed By: aria

Subscribers: cameron, eater

Maniphest Tasks: T3021

Differential Revision: http://phabricator.khanacademy.org/D12596