Add user to existing subfolder if it exists

[TimCsaky]

Description

ticket: https://apps.nrs.gov.bc.ca/int/jira/browse/SHOWCASE-3773?

• when adding a subfolder (using createChildBucket endpoint), if sub-folder already exists in COMS db, instead of returning 409 collision, give current user permissions to that sub-folder.
• when creating subfolder only require CREATE permission on the parent.

this will for example in bcbox, allow an external partner only given permission to upload into a folder, ability to create new subfolder.. on which they will also only have CREATE permission.

Types of changes

Bug fix (non-breaking change which fixes an issue)

Checklist

• I have read the CONTRIBUTING doc
• I have checked that unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

[github-actions]

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