Installation target now upgrades packages
Tidy up promotions dashboard

* Make breadcrumbs consistent
* Refer to 'content blocks' rather than 'promotions' within
  messages/templates
Remove dead template
Add management command for forking Oscar's statics

This makes it easier for FEDs to get started styling a new Oscar
project.  It copies all of Oscar's statics into a specified folder and
checks whether the new folder is in STATICFILES_DIRS.

Fixes #463