Make iframe_install.html trailing slash optional (bug 1066552)

(The main goal is to avoid the redirect when we load the URL without
 the trailing slash. We could just remove it, but let's keep it for
 backwards-compatibility)
Merge pull request #2543 from diox/iframe-install-optional-trailing-slash

Make iframe_install.html trailing slash optional (bug 1066552)
Migrate decorators.py and tasks.py from apps/amo (bug 1042820)
Merge pull request #2531 from eviljeff/2014-09-08

Migrate decorators.py and tasks.py from apps/amo
Document functionality added in bug 1042850
Merge pull request #2550 from eviljeff/realemaildocs

Document functionality added in bug 1042850
use ES for feed filtering
make comm emails better when coming from system
Merge pull request #2540 from ngokevin/feedfilter

use ES for feed filtering (bug 1067492)
fix app groups (bug 1066601)
fix search regression