Add Campaigns module for grouping short links.

Let accounts organise links by promo or launch, attach or detach links, and view combined click totals from the sidebar.

app/Models/ShortLink.php

@@ -11,6 +11,7 @@ class ShortLink extends Model
 {
     protected $fillable = [
         'user_id',
+        'campaign_id',
         'source_app',
         'source_kind',
         'source_ref',
@@ -40,6 +41,11 @@ class ShortLink extends Model
         return $this->belongsTo(User::class);
     }
 
+    public function campaign(): BelongsTo
+    {
+        return $this->belongsTo(Campaign::class);
+    }
+
     public function clicks(): HasMany
     {
         return $this->hasMany(LinkClick::class);