Add optional "start a trial" in experimental free SaaS signup

What does this MR do?

An experiment to add optional "start a trial" in experimental free SaaS signup

• Behind a feature flag to be rollout carefully

Related to #251230

Testing on GDK

On your GDK rails console enable the flags:

Feature.enable_percentage_of_time(:onboarding_issues_experiment_percentage, 100)
Feature.enable_percentage_of_time(:trial_during_signup_experiment_percentage, 100)

Screenshots

Form	Trial	Thanks

start-a-trial-in-free-signup-2020-12-11_21.56.28

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team