Add checks on creating/editing discount codes

[shreyanshdwivedi]

Describe the bug

Currently their is no server side validation of data sent on creating/editing a discount code.
If we try to enter amount > the ticket price, it is accepted.
If we try to enter % greater than 100, it is accepted

To Reproduce
Steps to reproduce the behavior:

1. Go to Manage Events
2. Click on Tickets
3. Scroll down to Discount Codes and try creating discount code with wrong data
4. See error

Expected behavior

A check should be implemented on -

• Discount Amount
• Discount %

Additional context

Working on it