Default caps on MongoDB collections

We should place a cap on the maximum size of MongoDB collections, especially the System Messages collection.
## Expected Behavior

Graylog audit logs should not be allowed to fill up all available space in MongoDB.
## Current Behavior

Audit logs can grow to the point that MongoDB fails for lack of space on the volume.
## Possible Solution

Cap max size for Mongo Collections by default.
## Steps to Reproduce (for bugs)

1.
2.
3.
4.

## Context

## Your Environment

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