Kill DebitAccountMiddleware.
Merge branch 'develop' into feature/issue-818-kill-DebitAccountMiddleware

Conflicts:
	go/vumitools/middleware.py
Kill CreditManager.
Merge branch 'develop' into feature/issue-818-kill-DebitAccountMiddleware
Nuke obsolete context processor.
Merge branch 'feature/issue-818-kill-DebitAccountMiddleware' into develop