Add a dedicated stylesheet

[edan-bainglass]

This PR moves CSS styles to a local stylesheet and loads it in the main notebook. It also adds more CSS classes for app containers. See aiidalab/aiidalab-qe#983 for an example.

Note that for feature logos, other than images (local or url-linked), you can also use Font Awesome icons.

For example, you can use an <img> tag:

<a
  class="feature"
  href="{appbase}/my_notebook.ipynb"
  target="_blank">
  <img
    class="feature-logo"
    src="{appbase}/<image-folder>/image.png"  -> can also use a link to an online image
    alt="My feature" />
  <div class="feature-label">My feature</div>
</a>

or an <i> tag for Font Awesome icons

<a
  class="feature"
  href="{appbase}/my_notebook.ipynb"
  target="_blank">
  <i class="fa fa-whatever feature-logo"></i>
  <div class="feature-label">My feature</div>
</a>

where fa-whatever is a Font Awesome 4 CSS class (ipywidgets 7 supports FA4)

[mikibonacci]

Hi @edan-bainglass, thanks for the work, LGTM!