Create a migration to insert trail plans

What does this MR do?

This migration helps in creating 2 plans for trials. One for ultimate and other for platinum. These ids would be used by the gitlab_subscriptions table which will help us organize the limits for the namespaces instead of providing a ultimate plan for the trial.

Up Migration

rake db:migrate
== 20210329102724 AddNewTrailPlans: migrating =================================
== 20210329102724 AddNewTrailPlans: migrated (0.0691s) ========================

Down Migration

rake db:rollback
== 20210329102724 AddNewTrailPlans: reverting =================================
== 20210329102724 AddNewTrailPlans: reverted (0.0795s) ========================

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • [-] I have not included a changelog entry because _____.
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• [-] Separation of EE specific content

Availability and Testing

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

Related to #322043 (closed)