I noticed the doc/development/testing_guide/best_practices.md still
referenced the `[run mysql]` tags, etc. They no longer work, so I
removed them, then realised I had better clean up the rest of
doc/development !

Code blocks should not be spaced 4 times, as this will
prevent the code from being colored. They should
also be spaced the same as the lists they are a part
of, to make reading easier.

We also enable the rubocop that makes it mandatory

This allows us to call `find_or_create_by` on all models and scopes.

Extending from ActiveRecord::Migration is deprecated, but was still used
in a bunch of places.

[ci skip]