feat: 🏅 Add alias routes for users and restaurants, validate inputs

[Volqano]

• Added alias for GET /users/{user_id} which can also be accessed via /users/{username}
• Added alias for GET /restaurants/{restaurant_id} which can also be accessed via /restaurants/{restaurant_name}. For restaurant names with spaces, use '_' in the URL.
• Integrated validation functions within User and Restaurant resources to ensure data integrity.
• Removed obsolete app.py and templates directories as they are no longer needed.

Solved Issues:

67 #84

[JanKaczmarski]

Moved to new repo https://github.com/HIM-Services/heaven-in-mouth