Add cypress UI tests

[thomaseizinger]

This PR adds cypress, a UI testing framework to our application.

The following screens are now covered with UI tests:

• [x] Wizard for creating a new nobt
• [x] Empty homescreen
• [x] Homescreen with items (1 bill)
• [x] Balances
• [x] Bill details (foreign & nobt currency)
• [x] Add bill

Open tasks:

• [x] Use a data-cypress attribute to select fields
• [x] Use baseurl config

Open questions (@duffleit):

~- Should we split out the sub-screens into individual files aswell? Currently, balances for example also covers the screens of each individual person and the test for the add-bill page covers stuff like currency conversion and selecting debtees/debtors. This makes the tests quite big ...~

I decided against it.

[duffleit]

Find ich ja mal mega-affen-titten geil dieses zeug. :100: #awesomenessoverload