create permission role to add/edit/delete extractors

## Expected Behavior
Be able to create permissions that would allow a user to create, edit and delete extractors on inputs.

## Current Behavior
As seen in https://github.com/Graylog2/graylog2-server/issues/4039 it is not possible to create (in a supported, working way) a user/role that is able to work on the extractors of a stream without admin rights.