feat: Add CCO's SSO to decidim-app

[AyakorK]

🎩 Description

Add the SSO of CultuurConnect to the decidim-app in order to retrieve this instance from a customized repository to the global app.

📌 Related Issues

• Ajout stratégie SSO compatible avec omniauth2 sur la decidim-app

Testing

Describe the best way to test or validate your PR.

Example:

• Log in on system
• Edit the organization
• Add the SSO of CultuurConnect (ask for the credentials if you need them)
• Save the organization
• Go to main app
• Try to access the page of CultuurConnect's SSO

Tasks

• Add specs
• Update the CI
• Add the Oauth2 gem
• Add logos of CultuurConnect
• Add locales
• Add the initializer
• Add the strategy of CCO's SSO

📷 Screenshots

Please add screenshots of the changes you're proposing if related to the UI