remove coupon redemption during clear cart and update test cases

[Ex-11]added test cases(E-commerce Tab Instuctor Dashboard)

update data model max_length to 255

Add column to the list of coupons in the E-Commerce tab
Merge pull request #4186 from edx/asadiqbal08/adding_coupon_redemption_count

(WIP) Add column to the list of coupons in the E-Commerce tab