make change of the index set for 'all messages' stream possible (API/UI)

## Expected Behavior
It should be possible to change the index set that is used by the 'all messages' stream.

## Current Behavior
The 'all messages' stream is hardcoded bound to the first created index (the prefixed that is configured in the server.conf) and it is not possible to change the target.

The only option to change the target is modification of the object in the mongodb

## Possible Solution
make it possible to change the target of the stream via API and/or UI - or bind this to the indices default and make it change if you change the default.

## Context
After written down of https://github.com/Graylog2/graylog2-server/issues/4386 and double checked with @bernd I realized that my issue is not just a cosmetic issue but more a architectural decision that is made on implementation.

## Your Environment
* Graylog Version: 2.3.2