Add collapsing and expanding of text diffs

Summary:
This commit collapses areas of the diff that are unchanged. They can be
expanded if needed by clicking on them, similar to how the property
diffs work.

I'm not too sure about the definition of the BEFORE and AFTER constants
at the top, since now those are duplicated from another file, but I'm
not sure we have a nice way to share constants between modules at the
moment(?) that wouldn't just be putting them in another module
altogether, but that seems a bit heavyweight for what I'm trying to
accomplish. Opinions welcome.

Test Plan:
* Open testdiff.html
* See the collapsed text parts

Reviewers: alex

Reviewed By: alex

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D7005