Add URL parameter tracking to SaaS trials

Problem

As we continue to work to increase the rate at which users start trials we don't have a way to track which paths are generating the most trials by volume or by conversion rates.

Solution

We should add URL parameter tracking to SaaS trials. The tracking could be something as simple as glm_source/glm_content/ and the value would be dependent on the location. To start we could add URL parameter tracking to what we believe are some of the larger sources for SaaS trials. We would store this URL parameter field and associated value in a hidden field in the trial signup flow (#13233 (closed)) which would allow us to track trial volume and associated close rates.

Location	Image	Trial Source Value
Top right drop down		top-right-dropdown
Security paywall		security-paywall
Group billing (CTA located between group name and tiers) /groups/group111111/-/billings		Group-billing
User billing /profile/billings		user-billing
about https://about.gitlab.com/free-trial/		about-free-trial