Add grouping/"cosmetic grouping" to streams

## Expected Behavior
I'm just about to create 80+ streams for one device group that is sending logs. This will get very messy on the Stream page, so there should be some kind of grouping function present.
Either a **simple cosmetic one**, see this mock up for example:
Here you could edit the groups on the right and would be able to assign the streams to the individual groups (multiple selections possible) and decide whether it should be displayed in the default view or not.

Another idea would incorporate a **complete grouping system with parent-child-relations**. For example the stream *NTP* is a parent-stream of the child-streams *Stratum1*, *Stratum2*, *Stratum3* and *ServerNTP*. The *NTP*-stream is an aggregation of it's child-streams and acts like any other stream, so if a alert condition that is defined on the *NTP*-stream is fullfilled by any of the child streams (or a combination of these streams), a notification will be triggered as usual.

I recognize that implementing a parent-child-system is quite time consuming, but I think most Graylog users having bigger production setups with many streams would benefit from such a solution, compensating for the implementation time more than enough. It adds a lot of visibility and also a way to create different levels of abstraction. For example, you could segment your monitoring into multiple levels like:

1. Global Health Status
2. Department Health
3. Team Health
4. Device Group Health
5. etc.

Maybe special alerts could further support this idea of parent-child-relations (e.g. If x child-streams have active alerts, trigger an alert --> automatic escalation if a bigger problem occurs.)

## Current Behavior
Currently, Graylog does not have any grouping function and only allows for a text filter to search for streams. This can be seen as enough, but will and can get quite messy over time, especially if some well known users use the system 😆

## Possible Solution
Idea for the cosmetic variant:
Add an attribute to streams that can contain a arbitrary name or id of an group and the frontend will take care of sorting/grouping the streams into the correct tabs.

## Context
I'm just about to create 80+ streams for one device group that is sending logs. And this is only for one device group. So I came to this idea.

I somehow have in mind to have read something similar in the past as an issue for graylog, but I'm not sure and am not able to find anything. Please correct me if I'm wrong and there is already an issue/feature request for this.

## Your Environment

* Graylog Version: Graylog 2.3.0-rc.1
* Elasticsearch Version: 5.4.3
* MongoDB Version: 3.4.2
* Operating System: RHEL7
* Browser version: Waterfox 54, Firefox 54, Chrome 59