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.
What do we think about adding some comments here? From just reading the file, I'm having a hard time understanding what the elements represent and as a result what work is being done.