Minify media viewer only

This PR replaced #2161 and based on #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
```
This PR should be tested using html 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.