Update billing section to support GitLab.com trials

What does this MR do?

• Add the trial_ends_on column to the namespaces table in order to store the trial status.
• Updates the Namespaces API in order to support the new column
• Move some EE specific code to the ee directory

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

Do we need a CHANGELOG entry?

Why was this MR needed?

We need to show the user when the GitLab.com trial will expire or when it expired. Also we should only allow to generate a trial once per namespace.

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
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• EE specific content should be in the top level /ee folder
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

REF: https://gitlab.com/gitlab-com/customers-gitlab-com/issues/210