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.
yay tests!