feature(frontend): add initial frontend styling, min/max prices

[igorsimb]

Fixes https://github.com/igorsimb/mp-monitor/issues/39, https://github.com/igorsimb/mp-monitor/issues/40

• Added styling to item_list and item_detail pages
• Added the following properties to Item model:
  • max_price
  • max_price_date
  • min_price
  • min_price-date
• Added info about min/max price to item_detail page