Add "issues created" action to continuous onboarding design

What does this MR do?

Add "issues created" action to continuous onboarding experiments

Related #281022 (comment 545948509)

• No changelog because it's behind a feature flag.
• A & B variant of the experiment are competing against each other

How to test locally

• Enable one of the feature flags: Feature.enable_percentage_of_time(:learn_gitlab_b_experiment_percentage, 100) # or the a variant
• Signup with a new user
• Go to the "Learn Gitlab" or any other project
• Click on "Learn Gitlab"

Troubleshooting

Since we require the "learn-gitlab" project to be available, which gets created in a background-job, it could be that the project didn't get created. This is because background-jobs are sometimes stuck in development. What to do then:

• gdk stop
• ps aux | grep runsv
  • If there is any other process beside grep, you need to run kill -9 <pid>
• gdk start again and go through the sign up flow again.

Screenshots

• See "Create an issue" below

A variant	B variant

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because this is behind a feature flag .
• 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