Implement automatic_trial_registration experiment

What does this MR do and why?

This MR implements an experiment that direct qualified prospects in the standard registration flow to a trial by default.

The experiment only targets users that select My company or team and Create a new project on the sign_up/welcome page and are not going through the trial_registration flow (hence standard registration flow).

These tracking are added for measuring impact of this experiment:

context	action
user in control clicks on trial toggle	click_trial_toggle
user in control or candidate successfully submits on "/company" form	successfully_submitted_form
after group/namespace is created, a manual assignment event to capture namespace	assignment

Screenshots or screen recordings

Control	Candidate

How to set up and validate locally

1. Start local GDK simulating SaaS.
2. In rails console, enable the experiment by Feature.enable(:automatic_trial_registration)
3. Go through register and on sign_up/welcome page, select My company or team and Create a new project
4. Land on the /sign_up/company form page and observe.
5. Finishing the registration locally requires setting up CustomersDot
6. Land on Learn GitLab with a trial for the group created during registration.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #404694