Resource name uniqueness not consistently enforced

## Expected Behavior

Graylog should enforce unique names for all resource types.

## Current Behavior

Graylog does not consistently enforce resource name uniqueness. For example:

Identically-named streams can be created:

Identically-named event definitions can be created:

Identically-named pipelines _can't_ be created, as the following occurs:
`Creating pipeline failed with status: FetchError: There was an error fetching a resource: . Additional information: Write operation error on MongoDB server $MONGODB_SERVER. Write error: WriteError{code=11000, message='E11000 duplicate key error collection: graylog.pipeline_processor_pipelines index: title_1 dup key: { title: "ghi" }', details={}}.`

## Steps to Reproduce (for bugs)

1. Create multiple resources of a certain type (e.g. stream) with the same name.
2. Note Graylog generally allows the identically-named resources to be created.

## Context

Creation of identically-named resources is almost certainly a mistake, and may occur due to various causes (e.g. simply overlooking something or multiple people creating things). Having identically-named resources can lead to confusion when working in Graylog.

## Your Environment

* Graylog Version: `7.0.3`