Refactor preloader

* make it simple (use only one DOM element for preloader)
* remove useless test (it tests nothing)
* unify preloader import/export

frontend/app/components/preloader/index.ts

@@ -1,3 +1,3 @@
-export { default } from './preloader';
+export { Preloader } from './preloader';
 
 // all styles were moved to iframe.html

frontend/app/components/preloader/preloader.test.tsx

@@ -1,12 +0,0 @@
-/** @jsx createElement */
-import { createElement } from 'preact';
-import { mount } from 'enzyme';
-import Preloader from './preloader';
-
-describe(`<Preloader />`, () => {
-  it('should render Preloader', () => {
-    const element = mount(<Preloader mix="root__preloader" />);
-
-    expect(element.childAt(0).hasClass('preloader root__preloader')).toEqual(true);
-  });
-});

frontend/app/components/preloader/preloader.tsx

@@ -1,17 +1,9 @@
 /** @jsx createElement */
-import { createElement, JSX } from 'preact';
+import { createElement, FunctionComponent } from 'preact';
 import b, { Mix } from 'bem-react-helper';
 
-type Props = JSX.HTMLAttributes & {
+interface Props {
   mix?: Mix;
-};
+}
 
-const Preloader = (props: Props) => (
-  <div className={b('preloader', props)}>
-    <div className="preloader__bounce" />
-    <div className="preloader__bounce" />
-    <div className="preloader__bounce" />
-  </div>
-);
-
-export default Preloader;
+export const Preloader: FunctionComponent<Props> = ({ mix }) => <div className={b('preloader', { mix })} />;