Allow naming of horizon instances

[timacdonald]

We have several horizon instances running in different regions all within a single application.

When we get notifications from Horzion, we are unable to determine which instance of horizon the issue is occurring in.

Long Wait Detected
[Nightwatch] The "{queue}" queue on the "{connection}" connection has a wait time of {seconds} seconds.

The above notification could be any region and we have no way of knowing.

With this PR, we are able to name our Horizon instances while maintaining a single shared app name.

Long Wait Detected
[eu-central-1] The "{queue}" queue on the "{connection}" connection has a wait time of {seconds} seconds.

Long Wait Detected
[us-east-2] The "{queue}" queue on the "{connection}" connection has a wait time of {seconds} seconds.

The default prefix also uses the APP_NAME, e.g., but I didn't want to break anything by changing this to respect the new var. It is right there in the config file for people to modify anyway.

    'prefix' => env(
        'HORIZON_PREFIX',
        Str::slug(env('APP_NAME', 'laravel'), '_').'_horizon:'
    ),

[timacdonald]

Backfills the value on existing installs