Koel can now integrate and use the rich information from Last.fm. Now
whenever a song is played, its album and artist information will be
queried from Last.fm and cached for later use. What's better, if an
album has no cover, Koel will try to update its cover if one is found on
Last.fm.
In order to use this feature, users only need to provide valid Last.fm
API credentials (namely LASTFM_API_KEY and LASTFM_API_SECRET) in .env. A
npm and gulp rebuild is also required - just like with every update.
The comment below motivated me to find a better solution for the
repeated authorisation checks:
```php
// This can't be put into a Request authorize(), due to Laravel(?)'s
limitation.
```
This is the result.
