fix bug 1051877 - Launch signin windows/links when icons within header login when clicked

Also:
-  Remove 'persona-loaded' CSS class from Persona buttons as we no longer lazyload Persona
-  Use JavaScript best practices for Persona launch link (avoid `javascript: ...` as the `href`)

Please spot-test:
- [x] Logging in with Persona via the sign-in page and via the header widget
- [x] Ensure optimizely + analytics events are sent
- [x] Ensure the "next" redirect functionality works as intended

Let me know if there are questions.
Loading Persona before our stuff, unfortunately, so that we aren't using stuff (auth) that isn't there.  We're also showing the login widget by default so the quicker we get the Persona loaded, the better.