enable / disable Elasticsearch Shard Allocation via GUI

## Expected Behavior
be able in the Graylog GUI to see the Status of the Shard Allocation Setting and enable/disable it in the GUI.

## Current Behavior
The following work on the cli is needed to enable/disable

```
curl -XPUT 'localhost:9200/_cluster/settings' -d
'{ "transient":
{ "cluster.routing.allocation.enable" : "all"
}
}'
```

```
curl -XPUT 'localhost:9200/_cluster/settings' -d
'{ "transient":
{ "cluster.routing.allocation.enable" : "none"
}
}'
```

## Context
If you are going into maintenance of the Elasticsearch Cluster, do a planed downtime/restart it would be nice to be able to check the cluster allocation like you can check the LB settings for Graylog. This would take of the need to check that on the CLI and it would be easy to spot if the allocation is disabled and unassigned shards happen to the cluster. Most Operators did not recall that they had disabled shard allocation before and run into this issue.