Feedback on failing authentication

[jejoivanic]

Description

This PR provides:

• A way for the user to handle the error response from the authentication request and display whatever the user wants in case of failure.
• Unit tests and e2e tests have been implemented.
• For the default AuthenticationLayout, added snackbar on failing authentication.

Closes #180

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Apart from running all previous tests (unit/e2e) two different e2e tests were added. Those are

• Testing snackbar visibility and content on failing authentication for invalid username
• Testing snackbar visibility and content on failing authentication for invalid password

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[jejoivanic]

Why is this a WIP?

I implemented a solution which requires the user to handle the asyn response from the authentication with a then. In this then's callback the user receives a parameter which represents the error of the response if any. If not, the layout will change and nothing should be received in this parameter.

...
  this.va.login(user, password)
    .then(err => {
      if (err)
        doSomething()
    })
...

I've thought about this solution but I don't really know if this is the right approach for this, however I could not find a better way. I've consider passing also the user, to resemble other libraries which, in similar cases, defines a callback like (data, err) => { ... }.
That's my only concern about this solution. I'll wait for some comments or feedback.

[jejoivanic]

I also modified the response from the demo server to return 401 status when the user or password is incorrect.

[sgobotta]

Not really sure what's going on. My guess is that the snackbar disappears before Cypress finally finds it in the virtual dom.

Note: this only happens during the CI, local e2e tests pass successfully.

[sgobotta]

Keeps failing, even with a timeout of 0. I'm going to temporarily remove those tests. It's not a big trouble since the plan is to move the demo to another repository, therefore e2e tests will not be part of the source code anymore.

[sgobotta]

End to end tests for AuthLayout snackbar were removed in 3326fe1