Flag to hide Fleet Desktop, even if it's bundled

[zhumo]

User story

As a Fleet admin, I want to hide Fleet Desktop even though it is bundled with Fleetd, So that I can roll out Fleetd first, observe the impact on my organization silently, and then, when ready, turn on Fleet Desktop later without re-installing the agent.

Requirements

• Allow the Fleet admin to turn off the Fleet Desktop icon from appearing to end users.
• By default, Fleet Desktop appears

Design

UI

TODO?

CLI usage

TODO?

Compatibility

REST API changes

TODO?

Database schema migrations

TODO?

Technical sub-tasks (if any)

N/A

[dherder]

@noahtalerman for context re: customer-rosner: https://fleetdm.slack.com/archives/C069QQKMMQU/p1722905958280699