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[bot]]

Codecov Report

:x: Patch coverage is 69.27374% with 55 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 87.31%. Comparing base (e3a1c5f) to head (ee22ad9).

:x: Your patch check has failed because the patch coverage (69.10%) is below the target coverage (95.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8638      +/-   ##
==========================================
- Coverage   87.37%   87.31%   -0.06%     
==========================================
  Files        1265     1266       +1     
  Lines       56429    56606     +177     
  Branches     2066     2066              
==========================================
+ Hits        49303    49424     +121     
- Misses       6618     6674      +56     
  Partials      508      508              

Flag	Coverage Δ
backend	89.00% <69.27%> (-0.08%)	:arrow_down:

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Components	Coverage Δ
Backend Apps	91.62% <ø> (ø)
Backend General	92.71% <69.10%> (-0.94%)	:arrow_down:
Frontend	70.07% <ø> (ø)

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[SchrodingersGat]

@matmair are you happy for me to push this to the next release target?

[matmair]

Let's move it, this will take a bit.