[idea] Add ability to hide action from action list

[homoky]

Some actions are used only when linked from another action so it might be cool to hide them from actions dashboard.

[alexarena]

Just like #7, this is going to be impacted by some big work we're doing on the dashboard front.

[jacobmischka]

This is now possible using the unlisted property in an action definition: https://interval.com/docs/writing-actions/#defining-an-action

Thanks as always for the feedback and suggestions!