This code extends the unsafe (typically user-supplied) HTML escape
built into Django to also escape angularjs templating markers. Safe
HTML will be unaffected.
Closes-bug: 1567673
Change-Id: I0cbebfd0f814bdf1bf8c06833abf33cc2d4748e7
(cherry picked from commit 4bc01cedf39cdeff2553d01cdace707a1ecf6620)