fix(metrics): Convert all `_` in screen names to `-`.

* Moved the screen name cleanup from lib/url.js to router.js, renamed fragmentToScreenName. The functionality makes more sense here.

@kparlante - this is for you!

fixes #1907
Merge pull request #1998 from mozilla/issue-1907-screen-name-metrics

fix(metrics): Convert all `_` in screen names to `-`.