Add migrations to enhance the association between Namespaces and GL.com plans

What does this MR do?

It modifies the DB schema to use a foreign key to associate a Namespace with a Plan instead of string the plan name as a String. Additionally it migrates the plan names to their corresponding Plan ID.

Are there points in the code the reviewer needs to double check?

We need to run a script in order to migrate all free users on GL.com to the Early Adopter plan, in order to do it I've created the following script: https://gitlab.com/gitlab-org/gitlab-ee/snippets/1670522

Why was this MR needed?

https://gitlab.com/gitlab-org/gitlab-ee/issues/2391#background

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #2391