Add button to hide/show subflow on dashboard page

[bsignoret]

On the dashboard page, we want to be able to hide subflows.
In general, when a subflow fails, the flow also fails. This overloads the dashboard interface when there are many subflows.

I suggest adding a button to show or hide subflows.
A change has already been made in prefect-ui-library: PrefectHQ/prefect-ui-library#2375

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[bsignoret]

Hello @pleek91 @znicholasbrown !

What do you think? Do you need more information?

Thank you

[znicholasbrown]

Apologies @bsignoret, this slipped the radar; this LGTM