fix: Add slug column to parties table and update seed data

- Added slug column to parties table via migration
- Updated seed data to properly assign slug values to Party records
- Fixed NoMethodError when assigning slug attribute in seed file

This resolves the ActiveModel::UnknownAttributeError that occurred
when trying to set the slug attribute on Party model instances.

app/controllers/parties_controller.rb

@@ -0,0 +1,11 @@
+class PartiesController < ApplicationController
+  # Display all events
+  def index
+    @parties = Party.includes(:user).upcoming.page(params[:page]).per(12)
+  end
+
+  # Display desired event
+  def show
+    @party = Party.find(params[:id])
+  end
+end