Do not count shared files on trashbin free space calculation

[VicDeo]

Description

Get free space directly instead of calculating it as quota - occupied

Related Issue

• Fixes https://github.com/owncloud/enterprise/issues/3610

Motivation and Context

Trash bin is emptied if the user has large files shared with him

How Has This Been Tested?

1. create user1 with 5 GB quota and user2 with 1 GB quota local storage
2. upload a 1.7 GB file to user1 and share it with user2
3. login as user2 and create a text file and delete that text file NOT THE BIG SHARED FILE!
4. set 'trashbin_retention_obligation' => 'auto,180',
5. run cron.php
6. look in logs.

Expected Result

the shared file should not count in the quota for user2 and the new created text file should remain in the trash bin of user2

Actual Result

The file is deleted from trash bin

{
  "reqId": "dV1TZBjg6TFOw4xok0Qu",
  "level": 0,
  "time": "2019-11-18T16:40:04+00:00",
  "remoteAddr": "",
  "user": "--",
  "app": "cron",
  "method": "--",
  "url": "--",
  "message": "Started background job of class : OC\\Command\\CommandJob with arguments : O:33:\"OCA\\Files_Trashbin\\Command\\Expire\":1:{s:39:\"\u0000OCA\\Files_Trashbin\\Command\\Expire\u0000user\";s:5:\"user2\";}"
}
{
  "reqId": "dV1TZBjg6TFOw4xok0Qu",
  "level": 1,
  "time": "2019-11-18T16:40:04+00:00",
  "remoteAddr": "",
  "user": "--",
  "app": "files_trashbin",
  "method": "--",
  "url": "--",
  "message": "remove \"carlos.txt\" (19B) to meet the limit of trash bin size (50% of available quota)"
}

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Database schema changes (next release will require increase of minor version instead of patch)
• Breaking change (fix or feature that would cause existing functionality to change)
• Technical debt
• Tests only (no source changes)

Checklist:

• Code changes
• Unit tests added
• Acceptance tests added
• Documentation ticket raised:
• Changelog item, see TEMPLATE

[codecov]

Codecov Report

Merging #36494 into master will decrease coverage by <.01%.
The diff coverage is 70%.

@@             Coverage Diff              @@
##             master   #36494      +/-   ##
============================================
- Coverage     64.68%   64.68%   -0.01%     
  Complexity    19024    19024              
============================================
  Files          1268     1268              
  Lines         74369    74369              
  Branches       1311     1311              
============================================
- Hits          48106    48104       -2     
- Misses        25877    25879       +2     
  Partials        386      386

Flag	Coverage Δ	Complexity Δ
#javascript	54.02% <ø> (ø)	0 <ø> (ø)	⬇️
#phpunit	65.86% <70%> (-0.01%)	19024 <4> (ø)

Impacted Files	Coverage Δ	Complexity Δ
apps/files_trashbin/lib/Quota.php	84% <70%> (-8%)	9 <4> (ø)

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[micbar]

LGTM