declare django as an extra dependency

[kalikaneko]

If possible, it'd be sweet to have the web api / admin being an optional install.
What do you think about declaring a panoramix[webui] extra for this purpose?

[kalikaneko]

maybe peewee is an alternative just for using the orm

[gkorf]

We are planning to split the code into two python packages: server side and client side. Django is a dependency for the server side only. A mixnet contributor or an end-user will just need the client code.