Hidden Thread component should allow setting a note and displaying content notices

[virtuous-cat]

Prerequisite: Move HiddenThread component from Frontend codebase to Components codebase BobaBoard/boba-frontend#68

• Add menuOptions, onSubmitHiddenThreadNote, onShowCns, isRevealed, onReveal, and editable props
• Show hidden thread note in body of hidden thread placeholder. This can be done with Editor as in the <TextSection> component.
• Add DropdownListMenu with options from props.menuOptions
• When note is editable, HiddenThread should have buttons to Save or Cancel
• Add show cn's button
• Add reveal button
• Add appropriate text and buttons when isRevealed = true

[essential-randomness]

Current design: