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/app/finders/software_license_policies_finder.rb

@@ -4,7 +4,7 @@ class SoftwareLicensePoliciesFinder
 
   attr_accessor :current_user, :project
 
-  def initialize(current_user, project, params = {})
+  def initialize(current_user, project)
     @current_user = current_user
     @project = project
   end
@@ -18,10 +18,4 @@ def find_by_name_or_id(id)
       software_licenses[:name].eq(id).or(software_license_policies[:id].eq(id))
     ).take
   end
-
-  private
-
-  def self.table
-    SoftwareLicensePolicy.arel_table
-  end
 end