Granular PAT demo

What does this MR do and why?

This is an MR to demo granular PATs.

How to set up and validate locally

1. In Rails console, create a granular PAT for a user:

   user = User.human.last
   token = PersonalAccessTokens::CreateService.new(
     current_user: user,
     target_user: user,
     organization_id: user.organization.id,
     params: { expires_at: 1.month.from_now, scopes: ['granular'], name: 'gPAT' }
   ).execute[:personal_access_token]
   project = user.projects.first
   scope = Authz::GranularScope.new(namespace: project.project_namespace, permissions: [:create_issue])
   Authz::GranularScopeService.new(token).add_granular_scopes(scope)
   puts "curl http://#{Gitlab.host_with_port}/api/v4/projects/#{project.id}/issues\?title\=My%20New%20Issue%20Title -X POST -H \"PRIVATE-TOKEN: #{token.token}\""

2. Copy the URL and execute it in another terminal instance. It should succeed and create an issue.
3. Run in console:

   puts "curl http://#{Gitlab.host_with_port}/api/v4/projects/#{project.id}/issues/#{project.issues.last.iid}\?title\=My%20Updated%20Issue%20Title -X PUT -H \"PRIVATE-TOKEN: #{token.token}\""

4. Copy the URL and execute it in another terminal instance. It should not succeed and output an error like:

   {"error":"granular_permissions_error","error_description":"Access denied: Your Personal Access Token lacks the required permissions: [update_issue] for \"alexbuijs/xxx\"."}

5. Try to access an as-of-yet unsupported API endpoint and it should return something like:

   {"error":"granular_permissions_error","error_description":"Unable to determine permissions for authorization"}