Decorator permissions should'nt be conditioned by STREAM_EDIT

## Expected Behavior

As a stream reader i would like to manipulate decorators since i have appropriate read/create/delete stream's decorator permissions.
## Current Behavior

As i'm simple stream reader, any decorator actions are forbidden.
## Possible Solution

If stream permission have to be check, for me only READ matters. As:
- we don't speak about stream lifecycle, i can't delete/edit stream
- we speak about custom fields useful for read aspect.
- i have appropriate decorators permissions.
## Steps to Reproduce (for bugs)
1. Create a stream
2. As administrator, give to user decorators:{stream_id}:read/edit/create stream:{stream_id}:read
3. As user, try to create decorators
## Your Environment
- Graylog Version: 2.1