fix bug 795356 - use sentry (errormill.mozilla.org)

0. `git submodule update --init --recursive`
1. Go to https://mana.mozilla.org/wiki/display/websites/Developer+Cluster#DeveloperCluster-Sentry to get the MDN dev SENTRY_DSN setting value
2. Add it to `settings_local.py`
3. trigger an error (I put `import something` at the top of apps/landing/views.py)
4. Go to https://errormill.mozilla.org/mdn/mdn-dev/ to see the error on errormill
5. eat cookies and/or drink beer