website: Enable CPEx on staging and development branches

[g0at1]

Context

Closes #4051. CPEx should be enabled on branches that are not production to facilitate better testing.

Implementation

I added new variable isProduction and use it to enable CPEx on non production branches.

Other Information

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Name	Status	Preview	Comments	Updated (UTC)
nusmods-export	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Apr 12, 2025 9:12pm

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 54.82%. Comparing base (988c6fd) to head (e0411db).
Report is 99 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4056      +/-   ##
==========================================
+ Coverage   54.52%   54.82%   +0.30%     
==========================================
  Files         274      276       +2     
  Lines        6076     6351     +275     
  Branches     1455     1556     +101     
==========================================
+ Hits         3313     3482     +169     
- Misses       2763     2869     +106     

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[jloh02]

Thanks for this update. However there's 2 problems that don't quite seem right to me

1. Doing this in featureFlags.ts doesn't seem right as this no longer becomes a feature flag
2. This implicitly breaks for production and the change away from a manual feature flag implies that it should be a fixed value

To solve this issue I would propose either of 2 alternatives:

1. Move this logic out featureFlags and convert the feature flag to something like "enableCPExForProd"; or
2. Create time based system for CPEx (which is difficult but preferred)

If we are keen on option 2 in this case (ie anyone deems the effort to reward ratio is worth to implement this), perhaps, opening a larger issue in place of #4050 and #4051 is better.