[bug 1087387, bug 1096541] Clean up feedback view code

* clean up the feedback view code, nixes silly code
* fix product/version/channel/platform inference code to be smarter and
  more correct
* set browser_platform where appropriate
* add tests for everything

Note: This hardcodes "Firefox dev" test which fixes the bug. However, we
should fix Response infer_* methods so they're using Product information
instead of being hard-coded. That's a project for a different bug....
Merge pull request #387 from willkg/1096541-feedback-view

[bug 1087387, bug 1096541] Clean up feedback view code