Introduce end-to-end testing

[DrumsnChocolate]

Since most of our apps don't change in terms of essential functionality, it would not be a terrible idea to add end-to-end tests for some key processes / use cases. For example, we can add tests for:

• login
• account activation
• debit collections
• permissions and more, if we see the need for it. I have some experience with nightwatchjs, and I think it would take some work to setup, but it could definitely provide some strong safeguards regarding PRs not breaking important stuff. Especially when we include it in CI actions as a requirement for merging a PR

[guidojw]

We used to have a lot of acceptance tests, those were removed in https://github.com/csvalpha/amber-ui/pull/44

Also, it makes more sense to use the testing tools that Ember goes hand in hand with: https://guides.emberjs.com/v4.4.0/testing