Allow ExCS admin to set starter project user_id

Specifically to set `Project#user_id` to `nil` to signify a starter (or
"public") project which can be viewed by anonymous users.

The logic in `Api::ProjectsController#project_params` is a bit hard to
follow. However, essentially we need to avoid the
`base_params.merge(user_id: current_user&.id)` line in the `else` branch
to avoid `Project#user_id` being set to the current user's ID.

Author: floehopper

app/controllers/api/projects_controller.rb

@@ -82,8 +82,8 @@ def load_projects
     end
 
     def project_params
-      if school_owner?
-        # A school owner must specify who the project user is.
+      if school_owner? || current_user&.experience_cs_admin?
+        # A school owner or an Experience CS admin must specify who the project user is.
         base_params
       else
         # A school teacher may only create projects they own.

spec/features/project/creating_a_project_spec.rb

@@ -218,6 +218,7 @@
           name: 'Test Project',
           locale: 'fr',
           project_type: Project::Types::SCRATCH,
+          user_id: nil,
           components: []
         }
       }
@@ -259,5 +260,12 @@
 
       expect(data[:project_type]).to eq(Project::Types::SCRATCH)
     end
+
+    it 'sets the project user_id to the specified value (i.e. nil to represent a public project)' do
+      post('/api/projects', headers:, params:)
+      data = JSON.parse(response.body, symbolize_names: true)
+
+      expect(data[:user_id]).to be_nil
+    end
   end
 end