Add an endpoint for Static Editor Editor

* Add controller, view and route
* Add a simple controller test

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/212558

app/controllers/projects/static_site_editor_controller.rb

+# frozen_string_literal: true
+
+class Projects::StaticSiteEditorController < Projects::ApplicationController
+  layout 'fullscreen'
+
+  prepend_before_action :authenticate_user!, only: [:show]
+
+  def show
+  end
+end

app/views/projects/static_site_editor/show.html.haml

+#static-site-editor{ data: {} }

config/routes/repository.rb

@@ -67,6 +67,10 @@ scope format: false do
       end
     end
 
+    scope controller: :static_site_editor do
+      get '/sse/*id', action: :show, as: :show_sse
+    end
+
     get '/tree/*id', to: 'tree#show', as: :tree
     get '/raw/*id', to: 'raw#show', as: :raw
     get '/blame/*id', to: 'blame#show', as: :blame

lib/gitlab/path_regex.rb

@@ -98,6 +98,7 @@ module Gitlab
       preview
       raw
       refs
+      sse
       tree
       update
       wikis

spec/controllers/projects/static_site_editor_controller_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Projects::StaticSiteEditorController do
+  let(:project) { create(:project, :public, :repository) }
+
+  describe 'GET show' do
+    let(:default_params) do
+      {
+        namespace_id: project.namespace,
+        project_id: project,
+        id: 'master/README.md'
+      }
+    end
+
+    context 'User roles' do
+      context 'anonymous' do
+        before do
+          get :show, params: default_params
+        end
+
+        it 'redirects to sign in and returns' do
+          expect(response).to redirect_to(new_user_session_path)
+        end
+      end
+
+      %w[guest developer maintainer].each do |role|
+        context "as #{role}" do
+          let(:user) { create(:user) }
+
+          before do
+            project.add_role(user, role)
+            sign_in(user)
+            get :show, params: default_params
+          end
+
+          it 'renders the edit page' do
+            expect(response).to render_template(:show)
+          end
+        end
+      end
+    end
+  end
+end