Fix relationship cleanup on deletion

[BenjaminCharmes]

Related to an issue in #1355

• When an item is deleted, all relationships pointing to it are now removed
• Prevents "nonexistent source" errors in the graph view
• Synthesis constituents are transformed to inline substances

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.17%. Comparing base (b121182) to head (bd0e960).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1368      +/-   ##
==========================================
+ Coverage   80.10%   80.17%   +0.07%     
==========================================
  Files          70       70              
  Lines        4755     4778      +23     
==========================================
+ Hits         3809     3831      +22     
- Misses        946      947       +1     

Files with missing lines	Coverage Δ
pydatalab/src/pydatalab/routes/v0_1/items.py	86.93% <100.00%> (+0.56%)	⬆️

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[cypress]

datalab    Run #3923

Run Properties:   Passed #3923  •  f6896714ff ℹ️: Merge bd0e960ea99f5a662629e82b89412aed14a93545 into b121182e7a2c4ca1eebd268ed0b8...

Project	datalab
Branch Review	bc/relationships-bug
Run status	Passed #3923
Run duration	07m 06s
Commit	f6896714ff ℹ️: Merge bd0e960ea99f5a662629e82b89412aed14a93545 into b121182e7a2c4ca1eebd268ed0b8...
Committer	Ben Charmes
View all properties for this run ↗︎

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Test results
Failures	0
Flaky	0
Pending	0
Skipped	0
Passing	336
View all changes introduced in this branch ↗︎