[3.x] Allow avatar path customization

[derekmd]

Covers: #742

Features

✔️ Apps can disable Gravatar and stop telling its parent Automattic about user activity on /telescope requests.
✔️ Existing apps still use Gravatar.
✔️ Apps can now show local avatars taken from your own image hosting rather than using a remote service.

Configuration

namespace App\Providers;

// ...

class TelescopeServiceProvider extends TelescopeApplicationServiceProvider
{
    public function register()
    {
        // ...

        Telescope::avatar(function ($id) {
            $user = User::find($id);

            return url("/assets/images/users/{$user->avatar_path}");
        });
    }

    // ...
}

Remote Service Example

To switch from Gravatar to Libravatar:

Telescope::avatar(function ($id, $email) {
    $hash = md5(Str::lower($email));

    return vsprintf('https://seccdn.libravatar.org/avatar/%s?s=200&d=%s', [
        $hash,
        urlencode(url('/assets/image/avatar-placeholder.png')),
    ]);
});

Implementation

The avatar URL is generated in PHP, only for JSON routes GET /telescope/telescope-api/{type}/{telescopeEntryId} for a preview of a single entry. i.e., this will not generate N+1 database queries when viewing a list of Telescope entries.

[taylorotwell]

Changed this so that just registering a callback with Telescope::avatar(fn) will enable custom driver. No need for configuration option.