[C2] Extend the user.auth_source field

[ywarnier]

The user table contains an auth_source field that stores the platform value by default.
If a user has to be authenticated through an extra authentication method, this field has to be changed to the authentication method.

However, in recent months, several users of Chamilo have reported that, in certain circumstances, you might want to offer the possibility for users to login via different, redundant mechanisms.

This would imply saving several auth_source for every single user,, which is not really an option at the moment.

Modify the user entity and all related resources to store the value of auth_source as an array rather than a simple string. This will allow us in the future to handle several authentication methods per user.

• convert entity
• create migration to transform the current values in auth_source to array
• verify import scripts
• verify backup/restore scripts
• verify cronjobs
• verify authentication methods and plugins to ensure compatibility