Telemetry should be disabled by default at the system level

## Expected Behavior

Telemetry should be disabled by default (i.e. opt-in) at a system level.

## Current Behavior

Currently (i.e. in the `5.1.0` release), `telemetry_enabled` defaults to `true` ( https://github.com/Graylog2/graylog2-server/blob/ce89737096a07ee7b7f7850bbb0baf2b7b0b8ec0/graylog2-server/src/main/java/org/graylog2/configuration/TelemetryConfiguration.java#L40 ). Without an admin changing that property to `false`, telemetry must be actively disabled on a per-user basis.

The in-app message (below) does not clearly indicate this data collection defaults to "on".

> We would like to collect anonymous usage data to help us prioritize improvements and make Graylog > better in the future.
> We do not collect personal data, sensitive information, or content such as logs in your instances.
> Learn more on our Privacy Policy .
> You can turn data collection off or on any time in the user profile.

Knowing what I was looking for, I searched the documentation and found it mentioned on the [Planning Your Deployment > Frequently Asked Questions](https://go2docs.graylog.org/5-1/planning_your_deployment/faq.html?tocpath=Planning%20Your%20Deployment%7C_____3) page-- not the most intuitive spot for something like this. The associated [5.1.0 changelog](https://go2docs.graylog.org/5-1/changelogs/changelog.html#Graylog510) item is very short.

## Possible Solutions

* `telemetry_enabled` (i.e. data collection) defaults to `false`
* `telemetry_enabled` is moved into a global runtime setting (defaulting to disabled), and admins have the ability to turn it on or off (perhaps initially with a one-time prompt)

## Context

Upgraded an environment to `5.1.0`, saw the in-app message, checked the user profile option, and saw it was enabled. I then tracked back the PRs involved to find the `telemetry_enabled` property (and later found the page mentioned above).

Support ticket #1628167428.

## Your Environment

* Graylog Version: `5.1.0`