BCN Integration with idCAT Mòbil

[NilHomedes]

Is your feature request related to a problem? Please describe.
Barcelona wants to integrate with Catalan e-ID (IDCat) in order to facilitate the registration through that electronic ID system.

Describe the solution you'd like
I want a feature that allows me to log-in and sign up in Decidim using IDCat. Once a user is registered using the ID Cat Oauth it should be verified.

Additional context
We could use the existing OAuth module developed by Generalitat de Catalunya: https://github.com/gencat/omniauth-idcat_mobil/tree/master

Does this issue could impact on users private data?
Yes, as it is a registration system.

Acceptance criteria

• Given I'm a visitor who wants to log in,
  When I access the log-in page,
  Then I can see a button labeled "Log in with IDCat".

• Given I'm a visitor,
  When I click on the "Log in with IDCat" button,
  Then I'm redirected to the IDCat authentication page.

• Given I have successfully identified myself through IDCat,
  When the authentication is completed,
  Then I am redirected back to Decidim,
  And I am logged in using my IDCat credentials.

• Given I have successfully logged in through IDCat,
  Then I'm verified.
  And I gain acces to restricted actions

• Given I fail to authenticate with IDCat (e.g., wrong credentials or system error),
  When IDCat returns an error,
  Then I see an error message explaining the issue,
  And I remain on the log-in page without being logged in.

[ferblape]

We already got the credentials from AOC, so we are ready to start on this.