Stream creation dialog is submittable when no index set is selected.

## Expected Behavior

When a user tries to create a stream and does not select an index set, the dialog is not submittable and any attempt to create the stream shows an error message indicating the problem instead and the dialog stays open.

## Current Behavior

When you try to create a stream and do not select an index set, clicking on "save" closes the dialog and just shows a temporary red error saying "Saving Stream failed with status: Error: cannot POST http://localhost:9000/api/streams (400)".

## Possible Solution

## Steps to Reproduce (for bugs)

1. Click on "Streams" in top level navigation
2. Click "Create Stream"
3. Enter Title & Description but do not select an index set
4. Click "Save"

## Context

## Your Environment

* Graylog Version:
* Elasticsearch Version:
* MongoDB Version:
* Operating System:
* Browser version: