Add user name and avatar on the top bar

[fzaninotto]

Requires a new method in the authProvider:

// in src/authProvider.js
const authProvider = {
    login: ({ username, password }) => { /* ... */ },
    getIdentity: () => {
        const { id, fullName, avatar } = JSON.parse(localStorage.getItem('auth'));
        return { id, fullName, avatar };
    }
    // ...
};

export default authProvider;

The change is backward compatible (if the authProvider.getIdentity() method isn't implemented, the admin shows an anonymous user avatar with no name, as before).

[djhi]

👌