Make role field multiple

Opened an initial MR for this feature.

There are 2 main open points to address related to this:

* Context for DashboardSectionStorage cannot be "role" anymore. Tried to find other situations where context could be multiple in core, but I was unable after a quick search. Not sure if we should explore that route, or just pass the user entity as context.
* We need a logic to determine what's the "default" dashboard in DashboardController. Currently it just renders the 1st one

Please check this proof of concept and let's iterate.

Dashboard assignment will be implemented based on a different approach.

Closing this.

Thanks for working on this anyway! Really appreciated :-)