Allow custom transparency link in Fleet Desktop

[zhumo]

Problem

Our customers want to change the transparency link to point to their internal resource re: employees' expectations about privacy and the agent's capabilities

Requirements

• Allow a user to specify a custom destination for the transparency link
• This should only be for Fleet Premium customers

Work

• In Fleet Desktop, add ability to use a custom URL for the "Transparency" link #5947
• Ensure new config item can be set via fleetctl apply #5949
• Add new fleet_desktop property to global config object #5408
• Add new "Fleet Desktop" section to global settings page #5950
• Create new transparency endpoint that responds with a 302 redirect to configured transparency URL #6200

[nonpunctual]

This should probably be closed right @noahtalerman? https://fleetdm.com/docs/using-fleet/fleet-desktop#custom-transparency-link

[noahtalerman]

Yep! We can close this. Thanks.

[fleet-release]

Custom link takes flight,
Guides through privacy's dense mist,
In Fleet's glass city light.