Add Bizible script to sign up pages

What does this MR do and why?

We'd like to hardcode Bizable into the product instead of via GTM to have a consistent implementation across our sign up flow platforms - about.gitlab.com, customers.gitlab.com, & gitlab.com. Calling Bizable via GTM also has data loss risk of 5-10%, so hardcoding it into the product eliminates that risk.

This MR adds the Bizible script to the following pages:

• https://gitlab.com/-/trials/new
• https://gitlab.com/-/trials/select
• https://gitlab.com/users/sign_up
• https://gitlab.com/-/trial_registrations/new
• https://gitlab.com/users/almost_there
• https://gitlab.com/users/sign_up/groups/new
• https://gitlab.com/users/sign_up/projects/new
• https://gitlab.com/users/sign_up/groups_projects/new
• https://gitlab.com/users/sign_up/welcome/trial_getting_started

Bizible would only run if the bizible config value is provided, and there is no current_user. This is also behind the long-lived ecomm_instrumentation feature flag, which will be turned on for gitlab.com only.

Closes gitlab-com/marketing/digital-experience/buyer-experience#211 (closed)

Related gitlab-org/charts/gitlab!2393 (merged)

Related omnibus-gitlab!5911 (merged)

Screenshots or screen recordings

How to set up and validate locally

Validate Bizible is not running

1. In rails console, make sure the feature flag is off

   Feature.disable(:ecomm_instrumentation)

2. Optionally comment out the bizible config value to your gitlab.yml file

   ## Bizible
   # bizible: false

3. Visit any of the listed pages above, such as http://127.0.0.1:3000/users/sign_in
4. Check that Bizible is not installed/invoked. Running Bizible in the Javascript console to get an undefined error or checking the network requests.

Validate Bizible running properly

1. In rails console, make sure the feature flag is enabled

   Feature.enable(:ecomm_instrumentation)

2. Add the bizible config value to your gitlab.yml file

   ## Bizible
   bizible: true

3. Visit any of the listed pages above, such as http://127.0.0.1:3000/users/sign_in
4. Check that Bizible is installed and running. Running Bizible in the Javascript console to get an object or checking the network requests.

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.