Use .should instead of .then to retry assertions in cypress

Our cypress tests have been a bit flaky lately. I think this is partially due to our usage of cypress' .then instead of .should. The cypress docs recommend using .should as it'll automatically retry:

As they note, we should be aware that the callback in a .should could be run multiple times, so we should make sure that there are no side effects in the callback.

So I'd say:

• If it's an assertion, use .should instead of .then (make sure there are no side-effects)
• If you want to work with the subject yielded from the previous command, and it's not an assertion, use .then

If everyone agrees I say we refactor our tests to follow the above.

Also a very good read: https://docs.cypress.io/guides/core-concepts/retry-ability.html. This explains exactly the problems we've been having. I think that this is important reading for everyone working with our cypress tests, as they expect tests to be written in a certain style.

This is partially addressed in #102.

After that it only leaves the tests in cypress/integration/forms that still have to be refactored. On that note, the tests in forms have a lot of calls to .wrap. I've found that moving tests to use .should turns them a bit flaky if there are calls to .wrap in there. We should move those out whilst refactoring these tests.

This will be addressed in #103