Added minimal template dropdown to item editor

Summary:
Added an [Insert template...] dropdown next to the [Add a widget...]
dropdown. This dropdown can insert boilerplate for tables, aligned equations,
and piecewise functions.

Test Plan:
Visit the item editor, try inserting all three templates. Try inserting
the table template with various numbers of newlines before it, and make
sure that there is always a blank line before the inserted table, but
that extra newlines aren't created when the table is inserted after an
already-existing blank line.
Test that inserting widgets is still functional.

Reviewers: alex, alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D4073