Create onboarding namespace action table

What does this MR do?

This adds the namespace_onboarding_actions table which will be used to record certain actions that happen within a namespace like "performed first git push", "created first MR", "started trial". We will use this to optimize the onboarding process for namespaces.

See: gitlab-org/growth/team-tasks#247 (closed) for more example actions. This MR is just introducing the table, I will create the different actions outlined in the issue in a follow-up MR.

Migrate

== 20201118093135 CreateNamespaceOnboardingActions: migrating ===========================
-- create_table(:namespace_onboarding_actions)
   -> 0.0093s
== 20201118093135 CreateNamespaceOnboardingActions: migrated (0.0108s) ==================

Rollback

== 20201118093135 CreateNamespaceOnboardingActions: reverting ===========================
-- drop_table(:namespace_onboarding_actions)
   -> 0.0031s
== 20201118093135 CreateNamespaceOnboardingActions: reverted (0.0051s) ==================

Screenshots (strongly suggested)

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