Add existing component to unit

[sdaitzman]

Screenshots

Figma workflow

Spec

• Users can add existing components to the unit

In Scope

• Add Content button at top of page opens Add Content sidebar
• User can select an existing block of their chosen type, confirm, and it appears within the active unit
• If users cancel by closing the Add Content sidebar or clicking cancel, no content is created
• Functional Add Existing Component button at bottom of unit beneath last component
• Add new/existing block is displayed inline in empty units as a quick shortcut to create a first block, or add an existing one (pending Add new/existing block interface for new units (high fidelity) #1607, else may be shifted to a future enhancement story)

Not in scope

• Full add-content sidebar overhaul shown in screenshots, that will likely fall under a future sidebars epic
• Updates to UI to show clearly that a component exists in multiple places (should fall under another story, since this will allow a component to be added to multiple units)

Additional States

(Add existing tab is at lower fidelity in this screenshot, and will need some updates for this story)