strip() strings from frontend and fix coupon validation error

fixes #24 (closed)

modified: make_post_sell/models/cart.py modified: make_post_sell/views/shop.py

Summary by CodeRabbit

• Bug Fixes

  • Adjusted coupon comparison logic to handle cases where cart_qualifier is None.
  • Removed leading and trailing whitespaces from input fields across various functions to ensure cleaner data handling.

• Improvements

  • Enhanced data processing by stripping unneeded whitespaces from product titles, descriptions, and prices.
  • Improved user input handling in shop and user settings by stripping whitespace and splitting email invitations into lists.