Fix incorrect prefix used in new uploads for personal snippets

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24550 fixed the
case where the wrong path on disk was being searched, but it
inadvertently ommitted the `/uploads/-/system` prefix when rendering the
Markdown for personal snippet uploads when they were stored directly in
object storage.

A personal snippet path is stored using FileUploader#upload_path.
The format for the path:

Local storage: :random_hex/:filename.
Object storage: personal_snippet/:id/:random_hex/:filename.

upload_paths represent the possible paths for a given identifier,
which will vary depending on whether the file is stored in local or
object storage. upload_path should match an element in upload_paths.

base_dir represents the path seen by the user in Markdown, and it
should always be prefixed with uploads/-/system.

store_dirs represent the paths that are actually used on disk. For
object storage, this should omit the prefix /uploads/-/system.

For example, consider the requested path
/uploads/-/system/personal_snippet/172/ff4ad5c2/file.png.

For local storage:

base_dir: uploads/-/system/personal_snippet/172
upload_path: ff4ad5c2/file.png
upload_paths: ["ff4ad5c2/file.png", "personal_snippet/172/ff4ad5c2/file.png"].
store_dirs: {1=>"uploads/-/system/personal_snippet/172/ff4ad5c2",
             2=>"personal_snippet/172/ff4ad5c2"}

For object storage:

upload_path: personal_snippet/172/ff4ad5c2/file.png

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/61671

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