Drop create_migration.py helper

The helper set up the environment for tests and then created the
migration. This can lead to subtle bugs if the test environment is
overriding models etc.

The helper is dropped to make creating migrations more explicit.
Remove custom user app from sandbox

This is to keep the sandbox as vanilla as possible, as it's the
environment that migrations should be built against.
Docs: Document how to create migrations
# Last 5 commit messages
# ----------------------
# 30eead5 Remove custom user app from sandbox [Maik Hoepfel]
# ee3ee02 Drop create_migration.py helper [Maik Hoepfel]
# a77e78c Don't show country field if only one is available [David Winterbottom]
# b5790aa Handle products without a category in GA subission [David Winterbottom]
# 39e0932 Merge pull request #782 from tangentlabs/feature/add_eway_extension_to_docs [David Winterbottom]
Avoid duplicate log messages when importing
# Last 5 commit messages
# ----------------------
# a52aaec Docs: Document how to create migrations # Last 5 commit messages # ---------------------- # 30eead5 Remove custom user app from sandbox [Maik Hoepfel] # ee3ee02 Drop create_migration.py helper [Maik Hoepfel] # a77e78c Don't show country field if only one is available [David Winterbottom] # b5790aa Handle products without a category in GA subission [David Winterbottom] # 39e0932 Merge pull request #782 from tangentlabs/feature/add_eway_extension_to_docs [David Winterbottom] [Maik Hoepfel]
# 30eead5 Remove custom user app from sandbox [Maik Hoepfel]
# ee3ee02 Drop create_migration.py helper [Maik Hoepfel]
# a77e78c Don't show country field if only one is available [David Winterbottom]
# b5790aa Handle products without a category in GA subission [David Winterbottom]
Correct a few template references

- basket_content.html: add form.instance to {{ product.get_title }}
- order.html: access to shipping_address without `order`
- order.html: bad ref: 'order.shipping_incl_Tax' (upper Tax)

Fixes #787