Minify media viewer only

This PR replaced #2161. Created on top of #2194.
Those changes allows to embed image viewer on the static HTML website. All static files are minified using django-pipline and served on:
```
http://HOST/static/omeroweb.viewer.min.css
http://HOST/static/omeroweb.viewer.min.js
```

To test it remotely please go to https://github.com/aleksandra-tarkowska/openmicroscopy/tree/minify_media_viewer_only/components/tools/OmeroWeb/omeroweb/webtest/templates/webtest/embeding and download ```embedded.html```. Then replace ```http://localhost``` in the following:

```
<link rel="stylesheet" type="text/css" href="http://localhost/static/omeroweb.viewer.min.css">
<script type="text/javascript" src="http://localhost/static/omeroweb.viewer.min.js"></script>
```
with the appropriate remote server name including custom prefix like ```http://remote_host/omero/```

If you wish to test this PR locally you can use static page in ```webtest/templates/webtest/embed```. Double click on the file and see if you can view image in the viewer.

In order to minify you need to run ```bin/omero web start``` that runs collectstatic command, your CSS and your javascripts will be compressed in the same time. If you run your development server directly you need to run ```python manage.py collectstatic``` first.