Revert "Use string notation for factory boy factories"

This reverts commit f243314.

I mistakenly assumed that the string notation would delay the loading of
the model class as it does e.g. when used for ForeignKeys or in the
AUTH_USER_MODEL. But in fact it achieves the opposite, it not just loads
the model, but it requires the entire model registry to be present!

This has been reported in
FactoryBoy/factory_boy#160

Until then, we need to stick to the old syntax.

Bump version to 0.8-beta-2

Bump version to 0.8-beta-1

Bump version to 0.6.5 and update docs

Bump version to 0.7.2 and update docs

Bump version to 0.4.16

Bump version to 0.4.15

Bump version to 0.7.1 and update docs

Bump version to 0.7.0 final

Woop.

Improve the 0.7 release notes