Get models and migrations into line

[chris48s]

Closes #479

Having spend a bit of time on this, it turns out there are 2 things going on here:

1. Adding explicit related_name field (this will have been related to one of the django upgrades)
2. Changing default values/descriptions etc from b'value' to 'value' (which will be something I missed in the python 3 upgrade).

I've also added a CI check to catch this in future.

[coveralls]

Coverage remained the same at 92.532% when pulling 6474557 on 479-migrations into 4fae9b7 on master.