Remove Registration Time Limit from Event Model and Add it to Settings Model

[kushthedude]

Please see fossasia/open-event-frontend#3161

[kushthedude]

@iamareebjamal @CosmicCoder96 , I needed a bit of help regarding fossasia/open-event-frontend# 3161, For the following change we will need some changes in the server too :

• Remove the Expiry_Time from Event Schema and Add an Expiry_Time field in Settings Model.
• Change the field value in helpers/order.py for the status change of order.
• Change apib, And as for the new changes made by me

Is there any other change which I would need to make?