Utilize django-compress

hyperkitty already makes use of django-compress: all .js / .css files are bundled into one file, which is then compressed, after software upgrade and before the page is served. On html page load only this file is sent to the client. This leads to less data transfer between client and http server. However postorious does not use django-compress, even if it will have identical benefits.

• Utilize django-compress in postorius.