WIP: Add reference sources model

[matmair]

This PR adds a way to add arbitrary or restricted (text) references to any model.
Together with a minimal UI implementation, this enables linking external sources/IDs and internal models. This is a starting point that will allow the development of plugins that expose stable linked data import/exports or integrate more transparently into external systems.

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[codecov]

Codecov Report

Attention: Patch coverage is 70.55556% with 53 lines in your changes missing coverage. Please review.

Project coverage is 84.59%. Comparing base (987e027) to head (bb15aba).
Report is 15 commits behind head on master.

Files with missing lines	Patch %	Lines
src/backend/InvenTree/common/tests.py	53.12%	30 Missing ⚠️
src/backend/InvenTree/common/models.py	68.11%	22 Missing ⚠️
src/backend/InvenTree/common/serializers.py	94.11%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8638      +/-   ##
==========================================
- Coverage   84.64%   84.59%   -0.05%     
==========================================
  Files        1190     1191       +1     
  Lines       54297    54465     +168     
  Branches     2057     2057              
==========================================
+ Hits        45962    46077     +115     
- Misses       7828     7881      +53     
  Partials      507      507              

Flag	Coverage Δ
backend	86.00% <70.55%> (-0.07%)	⬇️

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