Inheriting from `dict` has some unexpected side effects that don’t
happen with plain object. The one we want to avoid right now is that
a dict doesn’t seem to implement `__dict__` in a normal way, which
is required by `werkzeug.utils.cached_property`.
There was a lot of repetetive wrapping code being repeated for every
conditional block in these files. Let’s wrap it around the whole
conditional bit once instead.
Implements https://github.com/alphagov/notifications-utils/pull/81
Handles addresses as multiple columns:
- in ‘Send yourself a test’
- in example CSV files
- in validating that a CSV file has recipients (eg at least an ‘address
line 1’ and ‘postcode’ column)
- when showing the contents of a CSV file
As few UI changes as possible, once we have the thing working end-to-end
we can think about how the UI might need to work differently.
