Define a new property `AccessToken.scopes` to contain the list of
scopes granted to this access token.

Define a new remote method metadata `accessScopes` to contain a list
of scope name required by this method.

Define a special built-in scope name "DEFAULT" that's used when
a method/token does not provide any scopes. This allows access
tokens to grant access to both the default scope and any additional
custom scopes at the same time.

Modify the authorization algorithm to ensure that at least one
of the scopes required by a remote method is allowed by the scopes
granted to the requesting access token.

The "DEFAULT" scope preserve backwards compatibility because existing
remote methods with no `accessScopes` can be accessed by (existing)
access tokens with no `scopes` defined.

Impact on existing applications:

 - Database schema must be updated after upgrading the loopback version

 - If the application was already using a custom `AccessToken.scopes`
   property with a type different from an array, then the relevant code
   must be updated to work with the new type "array of strings".

Make it easier to add method/app-level scopes in the future