Add Cypress tests

[nuno-agostinho]

Run unit tests in GitHub Actions using Cypress.

[nuno-agostinho]

Hi @ritamtbsilva! This PR adds unit tests and automatically runs them every time a new commit is submitted to GitHub:

• You can click in the tab "Actions" to see the results (including screenshots of each web page tested)
• After this PR is merged, every commit in this repository should have a green checkmark or a red cross, depending if the tests are okay or not. This is useful to know if a change is breaking something in the unit tests. Here's an example:

To run the unit tests, I am using Cypress via GitHub Actions and I can tell you more about it if you are interested. 😊

As this required a lot of trial and error directly in GitHub, instead of merging the PR as usual, you can "Squash and merge" (which will join all the commits in this PR into a single commit).

If you have any questions, I am here for you!