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