test(webapp): provide tests in CI

- add url and page as env variables to choice where to test and update the others
- To pass a variable to a job's service, they have to be declared in the "variables" global -> doesn't work (see: [#37])
- use of the deployed image to do E2E test instead of services of gitlab

web-app/cypress/e2e/fetch_process.cy.ts

@@ -2,7 +2,7 @@
 
 describe("Test fetch measurments button in the main page", () => {
   beforeEach(() => {
-    cy.visit("http://localhost:8080/");
+    cy.visit("https://app.mse.kb28.ch");
   });
 
   it("Fetch timeseries with valid user-room-device", () => {