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

Previously, an HTTP request for
`/uploads/-/system/personal_snippet/:snippet_id/:hash/:filename` would
look for an uploader of `PersonalFileUploader` class and use
`PersonalFileUploader#upload_paths` to search the datbase for one of the
following paths:

1. `:hash/:filename`
2. `uploads/-/system/personal_snippet/:id/:hash/:filename`

However, when the upload were stored in object storage,
`PersonalFileUploader#store_dirs` stored the path as:

`personal_snippet/:snippet_id/:hash`

The extraneous `uploads/-/system` prefix prevented the path from being
matched, and uploads in object storage would return a 404 error. Uploads
in local storage would work fine.

To fix this, we set the `#base_dir` properly so that `#upload_paths`
generates the right value for object storage. Note that this also makes
`#store_dirs` do the right thing in `FileUploader`.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/52595

/app/spec/uploaders/personal_file_uploader_spec.rb:3: warning: already initialized constant IDENTIFIER
/app/spec/uploaders/namespace_file_uploader_spec.rb:3: warning: previous definition of IDENTIFIER was here

This reverts commit 54a575f1, reversing
changes made to c63af942.

Move uploads to object storage

Closes #4163

See merge request gitlab-org/gitlab-ee!3867

Use uploads/system directory for personal snippets

See merge request !2123