feature(test): Add a `/clear` route that clears browser state.

* Used in the selenium tests instead of running `unsafeEval` or `eval`

fixes #1285
Cleanup - remove last `eval` from functional tests, remove unnecessary jshint directives.
Merge pull request #1310 from mozilla/issue-1285-selenium-without-eval

feature(test): Add a `/clear` route that clears browser state.