Track logistration form toggles and password reset form views

As per [ECOM-591](https://openedx.atlassian.net/browse/ECOM-591). This tracks user-initiated toggles of the login and registration forms (i.e., we don't track the initial form load), as well as views of the password reset form.

@AlasdairSwan, could you review this?
I'm accessing Segment's library at `window.analytics`, following the pattern used in [customizable_search.js](https://github.com/edx/edx-mktg/blob/b78f90b353c88cec8e8258da75903f480344a0ef/docroot/sites/all/modules/custom/edx/customizable_search/customizable_search.js). Would you prefer that I pass it into the function call, as we're doing with jQuery, Underscore, Backbone, and gettext?