WIP: cascading soft delete

[kwk]

PLEASE: DO NOT MERGE WHEN THERE'S ONLY ONE APPROVAL.

See https://github.com/fabric8-services/fabric8-wit/blob/12f42bbe747f33bf0e3fe485d35ff66ac021d613/docs/cascading-soft-delete.md#cascading-soft-deletes for a description of what this PR does and why.

In addition to adding the cascading soft-delete mechanism for our complete database, I had to make a few adjustments to some tables:

• Delete no longer needed work_item_link_categories categories that is no longer used since migration 106.

• Add missing foreign key constraint from comments to work_items item and delete all comments that don't reference a work item.

• Change foreign key from tracker_items and tracker_queries to trackers from ON UPDATE RESTRICT ON DELETE RESTRICT to ON DELETE CASCADE.

• Change foreign key from identites to users to ON DELETE CASCADE.

• Implements [4] When a workitem is deleted, all links associated with it are deleted of Delete workitem #2304

• Addresses Work items are not deleted when space is removed openshiftio/openshift.io#4382

[codecov]

Codecov Report

Merging #2324 into master will decrease coverage by 0.17%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2324      +/-   ##
==========================================
- Coverage   70.17%   69.99%   -0.18%     
==========================================
  Files         171      171              
  Lines       16625    16610      -15     
==========================================
- Hits        11666    11626      -40     
- Misses       3829     3861      +32     
+ Partials     1130     1123       -7

Impacted Files	Coverage Δ
remoteworkitem/jira.go	75% <0%> (-25%)	⬇️
remoteworkitem/scheduler.go	53.65% <0%> (-7.32%)	⬇️
gormsupport/postgres.go	39.28% <0%> (-3.14%)	⬇️
controller/workitem.go	78.27% <0%> (-2.91%)	⬇️
search/search_repository.go	76.44% <0%> (-1.14%)	⬇️
spacetemplate/importer/repository.go	73.33% <0%> (ø)	⬆️
migration/migration.go	69.28% <0%> (+0.11%)	⬆️

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