settings/base: Add more require settings as defaults
Rewrite all imports to import from ganeti_webmgr module.

To make this into a proper python package, and to use django-admin.py this is
necessary. We can now import any of ganeti_webmgr's modules and not worry about
pythonpath.
Use fully qualified imports for custom fields

Had to update migrations in order for the migrations to access custom fields
Use fully qualified module in urls