Finish order process

[ScriptyChris]

• implemented saving new orders to database via Order model. Completed Orders view, which presents either all clients orders (for seller user view) or only for currently logged in client. Client can be redirected to newly made order after it's created via form
• added product's availability property - which gets decreased relatively to each product's ordered units - to prevent infinite amount of product's units per order
• improved navigation UX on ProductDetails and Account pages
• added prompt, which lets newly registered user to be redirected to login page
• made cart to be cleared on logout
• fixed NotFound page throwing an error
• updated tests
• temporary disabled integration with PayU API [Implemented basic integration with PayU API #27], because available payment method has to be synced with the one chosen by client in order form and - in case of the 3rd party API failure - a fallback needs to be implemented