[BootstrapAdminUi] Add loader

[loic425]

We'll document this on a cookbook to create an admin dashboard with statistics.

[diimpp]

It's approved since it's a direct copy of functionality from main package, but I've few points about it:

1. Loader should been made into component, right now each usage specifies image each time.

  <div class="sylius-loader">
      <img src="{{ asset('build/admin/images/loader.svg', 'admin') }}" alt="Loading"/>
  </div>

https://github.com/search?q=repo%3ASylius%2FSylius+sylius-loader&type=code

2. SVG image hardcodes size, color and css animation inside file, which should happen outside via css or some icon component

--- <svg width="24" height="24" stroke="#23B99A" viewBox="0 0 24 24" xmlns="http://www.w3.org/2000/svg"><style>.spinner{transform-origin:center;animation:circle 2s linear infinite}.spinner circle{stroke-linecap:round;animation:cir 1.5s ease-in-out infinite}@keyframes circle{100%{transform:rotate(360deg)}}@keyframes cir{0%{stroke-dasharray:0 150;stroke-dashoffset:0}47.5%{stroke-dasharray:42 150;stroke-dashoffset:-16}95%,100%{stroke-dasharray:42 150;stroke-dashoffset:-59}}</style><g class="spinner"><circle cx="12" cy="12" r="9.5" fill="none" stroke-width="3"></circle></g></svg>
+++ <svg viewBox="0 0 24 24" xmlns="http://www.w3.org/2000/svg"><g class="spinner"><circle cx="12" cy="12" r="9.5" fill="none" stroke-width="3"></circle></g></svg>

<g class="spinner"> probably should be extracted as well.

Here is example how css animation can look via css.

3. We should use tabler icon if possible