Replace Grappelli with Django Jet?

[troeger]

Django Jet seems to have a more modern UI by default, supports the same relevant features, and is mobile-friendly. It also has in-built theming support, so some of our CSS hacks would become unneccessary.

http://jet.geex-arts.com/

[troeger]

First attempt in djangojet branch branch. Some obvious problems identified so far:

• Dashboard changes need a frequent 'migrate.py flush', since some heavy database caching seems to take place.
• pre_content trick we used in Grappelli for the general info list is no longer working. Might need a custom DashboardModule implementation that follows the Jet rules.
• Widget addition control in the upper right corner seems to be broken.

Beside that, the optics are really nice. Let's just wait for Django Jet to stabilize before continuing with that.

[troeger]

Django Jet would fix #135.