[OAuth] Client with multiple grant types?

[netsensei]

A client can have multiple grant types right now. i.e Implicit and Client Credentials. We haven't tested yet if the current codebase supports this use case. We also need to determine if this is a valid use case against the RFC specification.

Detailed description

It's possible to create / edit an OAuth client and select multiple "Allowed grant types". However, it's unclear what the consequences of this design choice is. We need to research this.

If this is not a feasible / desirable option, we need to revert this to a simple dropdown instead of multiple select form element. And make according changes in the data model.