feat: Make all order relationships read only

[iamareebjamal]

There is absolutely no reason to allow changes in order relationships and those were previously disallowed via custom logic. But for some reason, SQLAlchemy was failing with Unhashable type: 'dict' when saving custom forms JSON in attendee (working) and then saving the order (throwing) either due to a bug in SQLAlchemy or flask-rest-jsonapi (more likely). So now, we have made order relationships read only so that none of the related items are saved needlessly on PATCH request

[niranjan94]

Here is an overview of what got changed by this pull request:

Complexity decreasing per file
==============================
+ tests/all/integration/api/helpers/order/test_edit_order.py  -1
         

See the complete overview on Codacy

[codecov]

Codecov Report

Merging #7090 into development will increase coverage by 0.01%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           development    #7090      +/-   ##
===============================================
+ Coverage        61.99%   62.00%   +0.01%     
===============================================
  Files              262      262              
  Lines            12992    12992              
===============================================
+ Hits              8054     8056       +2     
+ Misses            4938     4936       -2     

Impacted Files	Coverage Δ
app/api/schema/orders.py	96.87% <ø> (ø)
app/api/orders.py	39.26% <0.00%> (-0.53%)	⬇️
app/api/helpers/permission_manager.py	36.49% <0.00%> (+0.70%)	⬆️
app/models/user.py	60.29% <0.00%> (+0.74%)	⬆️

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