fix: enable saving of subtechnique-of relationships

[adpare]

The frontend tried to create the subtechnique-of relationship before the new subtechnique itself had been saved. The relationship pointed to a child revision that did not exist yet, so the backend rejected it.

In this PR, for a new subtechnique, we save the child technique first, then create its subtechnique-of relationship and sync tactics afterward.

Related to PR open on the rest-api

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 0% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 47.28%. Comparing base (5f6b572) to head (e3ca922).

Files with missing lines	Patch %	Lines
src/app/classes/stix/technique.ts	0.00%	18 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##             beta     #960      +/-   ##
==========================================
- Coverage   47.98%   47.28%   -0.70%     
==========================================
  Files         208      208              
  Lines       27966    27980      +14     
  Branches     2082     2057      -25     
==========================================
- Hits        13419    13230     -189     
- Misses      14455    14659     +204     
+ Partials       92       91       -1     

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