Bug: Subscribe button not disabled when logged out

[duoi]

Hello,

Issue: Currently the subscribe button is not disabled when a user is logged out.

Impact: It's still possible to click the button, but it doesn't do anything or give any feedback.

Proposed solution:

1. Introduce isLoggedIn or similar to determine login state.

import { auth } from '../../store';
let isLoggedIn = false;
auth.subscribe((value) => {
    isLoggedIn = value !== null;
});

3. Apply this to both the subscribe and the add to playlist buttons (which currently just check for personalPlaylists being undefined).
4. Add login required tooltip when the disabled state is active

[WardPearce]

Fixed via #23