fix: Only admin should be allowed to create event discount codes

[prateekj117]

Fixes #6049

Checklist

• I have read the Contribution & Best practices Guide and my PR follows them.
• My branch is up-to-date with the Upstream development branch.
• The unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

• All the functions created/modified in this PR contain relevant docstrings.

Short description of what this resolves:

Any user was able to create event discount code.

Changes proposed in this pull request:

-Only admin should be allowed to create an event discount code.

[prateekj117]

@uds5501 @mrsaicharan1 @shreyanshdwivedi @iamareebjamal Please review.

[kushthedude]

Change the title of PR to Event Organiser of Event Admin,

[codecov]

Codecov Report

Merging #6050 into development will not change coverage.
The diff coverage is 50%.

@@             Coverage Diff              @@
##           development    #6050   +/-   ##
============================================
  Coverage        66.27%   66.27%           
============================================
  Files              285      285           
  Lines            14015    14015           
============================================
  Hits              9288     9288           
  Misses            4727     4727

Impacted Files	Coverage Δ
app/api/discount_codes.py	22.72% <50%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8f253f5...84f3910. Read the comment docs.

[uds5501]

@kushthedude , It's actually only admin. @prateekj117 is right 😅 .
The event discount code creation rights rest with admins , while ticket discount codes can be created by Event Organisers .

[kushthedude]

Oh my bad @prateekj117 , @uds5501 , I misunderstood it

[kushthedude]

@prateekj117 @iamareebjamal Should this PR be merged, As Event Discount Code is functional for now?

[iamareebjamal]

@CosmicCoder96 Please verify

[shreyanshdwivedi]

Just a note : we are not using the event discount code for now, but fixing the code LGTM 👍