Support Bearer Tokens for authenticating instead of using a token in basic auth

## Expected Behavior

When a user creates a token which can be used for authentication, it should be accepted by the server when passed as part of a `Authentication: Bearer ` header.

## Current Behavior

For token authentication, the server expects basic auth with the username set to the token and password to `token`. This is rather proprietary. Additionally, some systems which are otherwise capable of speaking to Graylog (e.g. the [telegraf prometheus plugin](https://github.com/influxdata/telegraf/tree/master/plugins/inputs/prometheus) speaking to the [Graylog prometheus metrics reporter](https://github.com/graylog-labs/graylog-plugin-metrics-reporter/tree/master/metrics-reporter-prometheus) do not work due to the nonacceptance of Bearer Tokens.

## Possible Solution

## Steps to Reproduce (for bugs)

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## Context

## Your Environment

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