Fixes #21403

Add a new configuration-entry to manage creation of access-tokens for external users.

Description

The configuration's value is considered when a user wants to create a new access token. For internal users, it doesn't make a difference, for external ones, it would forbid creating a token if set to false. Otherwise, external users are also allowed to create access tokens.

Starting with graylog 6.2, the configuration is set to not allow external users creating an access token. Before that version, it is allowed, to prevent a breaking change.

Motivation and Context

How Has This Been Tested?

There are test-cases covering all three options (internal user, external with setting being true and external user with setting being false).

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.