Refactor codebase to use hooks and Vanellus instead of actions and HOCs

[adewes]

Currently the app relies on higher-order components (HOCs) and custom actions to perform tasks. We want to change this so that the app just relies on hooks and the Vanellus functionality.

Currently there already are hooks that load the user, provider and mediator instances. So the user dashboard e.g. loads

// get the current user object
const user = useUser()

// load appointments
useEffect(() => {
    user.getAppointmentsById(...)
})

// display data from the user object
return <div>{user.secret}</div>