Support 'plus' button in top of app

[rollie42]

(Repurposing this ticket based on the figma)

Support the '+' button in the top left of the page; this should present the user with the ability to select one or more people in a modal dialog of all contacts. Probably this list will be in a new context of its own, something like const contactsContext = createContext<[User[], any]>([[], () => {}]) Just follow the way the other createContexts are set up; feel free to ask in channel or in mentor channel for help or someone to shadow :)