Add E2E tests

[colorfield]

E2E tests could cover the essential features so we are able to refactor in confidence.
They could probably be favoured to php unit tests as they are in general easier to maintain, language agnostic, covers the system as a whole and aims to stay close to user stories.

Cypress (https://www.drupal.org/project/cypress) could be used for that and configured for Travis or Circle CI, ...
As most of the pages are content based (e.g. paragraphs embedding views / features blocks, ...) we might need some fixtures and cannot rely on configuration only.

Proposal for test coverage:

• As an anonymous user I can create a new user account
• As an anonymous user when I login, I am redirected to the 'add celebration' page
• As an authenticated user I can add a video
• As an authenticated user I can add a photo
• As an authenticated user I can add an event
• As an authenticated user the media library is showing only my own media

Use case
As a developer
When I submit a pull request
Then I can check the system status

• E2E test solution is configured
• CI service is configured
• Fixtures are executed as a precondition
• Tests are being integrated