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

* Used in the selenium tests instead of running `unsafeEval` or `eval`

fixes #1285 

@zaach, @pdehaan or @vladikoff - wanna have a look?
Only for selenium tests? Wouldn't we want to route users through something like this for a /signout flow where we purge everything we've collected in session/local/cookie storage?