Add License Management API to the backend

What does this MR do?

It adds approval and blacklist API for software licenses; see https://gitlab.com/gitlab-org/gitlab-ee/issues/5488

Why was this MR needed?

To implement https://gitlab.com/gitlab-org/gitlab-ee/issues/5488

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?

Refs #5488 (closed)

ee/lib/api/managed_licenses.rb

@@ -6,6 +6,7 @@ class ManagedLicenses < Grape::API
     before { authorize! :admin_build, user_project }
 
     helpers do
+      # Make the managed license specified by id in the request available
       def managed_license
         @managed_license ||= user_project.managed_licenses.find(params[:managed_license_id])
       end
@@ -61,7 +62,6 @@ def managed_license
         success Entities::ManagedLicense
       end
       params do
-        #requires :managed_license_id, type: Integer, desc: 'The id of the managed license'
         optional :name, type: String, desc: 'The name of the license'
         optional :approval_status,
           type: String,
@@ -82,9 +82,6 @@ def managed_license
       desc 'Delete an existing managed license from a project' do
         success Entities::ManagedLicense
       end
-      #params do
-      #  requires :managed_license_id, type: Integer, desc: 'The id of the license'
-      #end
       delete ':id/managed_licenses/:managed_license_id' do
         not_found!('ManagedLicense') unless managed_license