Custom license templates

What does this MR do?

Adds an admin UI to add additional custom licenses to GitLab.

These licenses are made available as additional license choices when adding a license file in a project. The standard licenses are still provided in any case.

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

Why was this MR needed?

As GitLab is a promoter of innersourcing this MR enables adding custom license files.

The development of this MR is sponsored by @siemens (/cc @bufferoverflow)

Screenshots (if relevant)

New admin UI

Custom templates in the new license file dropdown

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?

Relates to https://gitlab.com/gitlab-org/gitlab-ce/issues/25631

Relates to https://gitlab.com/gitlab-org/gitlab-ee/issues/1602