Webclient: create custom role types (#680).

To create a custom role type use the Create menu's new item "New Role Type".
Enter a human-readable name and an URI.
Every role type needs an URI. A default URI is provided automatically.

Core Service API:

- `createRoleType()` is a new method

Webclient API:

- do_create_role_type() is a new controller method
- create_role_type() is a new storage method

REST Client:

- create_role_type() is a new method

See #680.