[bug 1074315] Ignore deleted files for linting in pre-commit hook

Use mythmon's fix for this issue in kitsune repository from
https://github.com/mozilla/kitsune/pull/2252
Merge pull request #408 from lgp171188/1074315

Fix bug "1074315 -  fix bin/flake8_lint.sh script to not try to lint files that don't exist"
Get list of .py files to lint using just git diff
Merge pull request #409 from lgp171188/improve-getting-list-of-py-files-for-lint

Get list of .py files to lint using just git diff
Add django-sslserver

This helps when doing development because it gives us https so we can
test privileged code.
[bug 1104935] Add remote.js

This library gives us access to the remote-troubleshooting data.
[bug 1104935] Add browser data section to generic form

* add browser data section to form
* tweak CSS to make it display a little less wonky
* add tests for making sure it only shows up for en-US and when
  feedbackdev flag is True
* add tests for verifying correctness of code

Note: This doesn't include smoketests because it requires https and
special preferences plus this data isn't exposed in the ui anywhere
after it's been collected. We'll have to test it by hand.