Change location of personal snippet uploads

We introduced uploads for personal snippets in https://gitlab.com/gitlab-org/gitlab-ce/commit/43ff7386411af0f538710f3627622f71e5e34472#2f154d17ee4bad51947893baa3c1671d74502ca6_0_12 when we implemented support of comments for personal snippets.

We started uploading snippets into public/uploads/personal_snippets but they should be uploaded to public/uploads/system/personal_snippets instead.

As part of !11071 (merged) we added description to snippets - another place where users can upload files for personal snippets. Also these uploads end in the wrong path.

We should change PersonalSnippetUploader so that files end up in the correct location, under system directory.

As part of this issue all existing uploads have to be moved to the new location.

While cleaning up the symlinks that kept the files available during the deploy, we missed cleaning up the appearance folder because of a typo.. The folder still needs to be cleaned up because if someone would create a group with full patch appearance/logo or appearance/header_logo the images stored in those locations could get lost.