Extensible user roles

[This question on StackOverflow](https://stackoverflow.com/q/52382306/857390) asks how to extend the list of roles a user can have in an organization (`admin`, `editor`, `member`). AFAIK this is currently not possible, but IMO would be a nice feature.

This could be done, for example, by an interface `IRoles` using which plugins could return a list of custom roles.

Start by reviewing the linked Stack Overflow question and the current CKAN organization-role implementation; this issue does not name files or tests. Determine how plugins currently extend organization behavior and define the supported custom-role interface. Done means custom roles can be declared and used alongside admin, editor, and member with documented behavior.

Written by the indexing model from the issue text.