[export] Only export as deleted entries explicitly so

[imnasnainaec]

If a word is removed from the frontier by some unforeseen bug, we don't want that word deleted from the user's FieldWorks project. So this pr will add a guard against that.

Required database update to go with this pr:

• Add Deleted copies of any words with GUID missing from the Frontier.

Reason: Prior to #4155, merge children were not marked as deleted in the database, so without the database update, this change would result in the children of merges not being deleted in FW, which would undermine the cleanup done in TC.

Side-effect: We cannot distinguish between words that were wrongfully lost from the fronter and words that were deleted (not merged) with the Merge Duplicates tool. So any of the former (which are so far only known to exist in one project), will be cemented as Deleted.

The database update will be easier after #4179 (and it's associated database update) is complete.

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This change is 

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.52%. Comparing base (5ebd071) to head (e3967c4).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #4178       +/-   ##
===========================================
+ Coverage   74.95%   86.52%   +11.57%     
===========================================
  Files         302       56      -246     
  Lines       11090     4846     -6244     
  Branches     1394      604      -790     
===========================================
- Hits         8312     4193     -4119     
+ Misses       2378      511     -1867     
+ Partials      400      142      -258     

Flag	Coverage Δ
backend	86.52% <100.00%> (+<0.01%)	⬆️
frontend	?

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