Broken uploads when moving issue to another project

It looks like we also need to move uploads when moving issue to another project.

Example:

https://gitlab.com/gitlab-org/gitlab-ee/issues/408#note_4395478

Moved from gitlab-org/gitlab-ee#410

Added ~14281 ~12980 labels

Milestone changed to 8.6

Added regression label

@grzesiek I labelled it regression since it's a sizable bug in a new feature

@DouweM Thanks, I will pick this one tomorrow, to ship fix as soon as possible.

mentioned in merge request !3382 (merged)

mentioned in issue #14303 (closed)

Status changed to closed by merge request !3382 (merged)

mentioned in commit 36c8506b

mentioned in commit 98df8aab