DEV: improves rails plugin generator (#7949)

Fixes bugs, simplifies code, more default files. General idea, more is more here as it's easier to just delete things than reading and passing all the options.

lib/generators/plugin/templates/assets/javascripts/discourse/adapters/adapter.js.es6.erb

@@ -0,0 +1,7 @@
+import RestAdapter from "discourse/adapters/rest";
+
+export default RestAdapter.extend({
+  basePath() {
+    return "/<%= dasherized_name %>/";
+  }
+});