Move Mongo Index Creations out of constructors into a separate Module

JAX-RS Resources by default are not singletons. This in some cases, causes a recreation of MongoDb Indexes for each incoming request.
## Expected Behavior

MongoDb indexes are created once during startup
## Current Behavior

MongoDB indexes are created for each incoming request. e.g. `/api/system/indices/index_sets/`

## Possible Solution

Suggest that after migrations are run, indexes are created.
## Steps to Reproduce (for bugs)

1. Set up tracing with JAEGER
2. Call /api/system/indices/index_sets/ and check traces
3.
4.

## Context

## Your Environment

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