Respect quota on export

[Pytal]

Exporting with contents that exceed the user quota proceeds without error currently

• test what happens when exporting and there is not enough space
  • is the file truncated ? is there an error ?
  • if not, might need to use the quota wrapper
  • I expect that it works because of https://github.com/nextcloud/server/blob/master/lib/private/Files/Storage/Wrapper/Quota.php#L171
  • need to detect that error case and report the error in a clean way

The following steps may be taken to solve this:

1. Use TempExportDestination as export destination in UserExportJob
2. When zip is fully exported
   • Create a new file in user folder with $userFolder->newFile() with the zip stream as content and check if quota is handled automatically
   • If quota is not handled automatically with $userFolder->newFile() then only create an export in the user folder if zip size + used space does not exceed the user quota