Rules in pipeline stage definitions should be referenced by a unique id not title

This is more of a UI usability problem than a real software bug.

Looking at at `api/plugins/org.graylog.plugins.pipelineprocessor/system/pipelines/pipeline`, we see that pipeline stage definitions reference rules by title and not their id:
```json
"stages": [
{
"stage": -1,
"match_all": true,
"rules": [
"Pass messages with Linux origin"
]
},
{
"stage": 0,
"match_all": false,
"rules": [
"Extract user name from SSH login",
"Extract RSA fingerprint from SSH login",
"Extract command parameters from sudo invocations"
]
}
]
```
Let's consider a rule A that is part of N different pipelines in S different stages (in any stage), where N is a bit bigger than very small say 10, and change just the title of that single rule, because the new title is better.

## Expected Behavior

The title should be changed and my many pipelines continue to work.

## Current Behavior

The stages in all pipelines where the rule is in break because the rule "has been renamed or removed. This rule will be skipped". I now get to go through all pipelines and fix the issue (rename) unless I employ a rule management script or similar automation through the API.

## Possible Solution

Link rules to stages by a unique object id, not the title which is shown in the user interface. This should remove a lot of friction in pipeline management through the UI.

## Your Environment

* Graylog Version: 2.2.2
* Elasticsearch Version: 2.4.4
* MongoDB Version: 3.4.2
* Operating System: Debian 8
* Browser version: Firefox 52.0