[ENG] Add tracking events for trial reassurance experiment

Track three key things:

1. Who is in the experiment & which variant they received – tracked by GLEX automatically, & we’ll also record to the experiment_subjects table once the person does sign up and becomes an actual User in our system
2. A frontend event when they initially land on the registration page
   • Event data will have useful things like which variant they received & what their viewport width was so we can know if they would have actually seen the new content or not
3. A second frontend event when they land on whatever page comes after they successfully start their trial
   • Event data will contain the same useful information as the previous FE event

Note: We may also want to track intermediate steps to get a better insight into the overall trial + registration flow funnel.