function persistAuthUserLocally(user) {
	const keys = ['uid', 'displayName', 'photoURL', 'isPro', 'settings'];
	const obj = {};
	keys.map(key => (obj[key] = user[key]));
	window.localStorage.setItem('user', JSON.stringify(obj));
}
Persists the firebase user with a subset of it's keys.
@param {object} user User object from firebase