We don't need to find the filename from the remote URL

When file_mover_spec.rb ran, it would initialize fog-aws with
`Fog::AWS::Storage::Real` service instead of `Fog::AWS::Storage::Mock`
because `Fog.mock!` was not called. Ensure that we use
`stub_uploads_object_storage` to prevent that from happening.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/64083

It persist temporary personal snippets under
user/:id namespaces temporarily while creating
a upload record to track it. If an user gets removed
while it's still a tmp upload, it also gets removed.
If the tmp upload is sent, the upload gets moved to
personal_snippets/:id as before. The upload record
also gets updated to the new model type as well.

We've already migrated all the legacy artifacts to the new realm,
which is ci_job_artifacts table.
It's time to remove the old code base that is no longer used.

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

String#underscore isn't particularly slow, but it's possible for us to
call it many times in a users autocomplete request, with mostly-static
values ('User', 'Group', etc.). We can memoise this and save a
surprising amount of time (around 10% of the total request time in some
cases).

Unlike uploads that have been uploaded with Tempfile, the project
import/export archives are stored in a temporary cache directory and
remain there if:

1. Object storage is enabled
2. `move_to_store` is set to `true`.

CarrierWave will leave these files there until disk space runs out or a
clean step is run manually.

If `move_to_store` is set to `false`, CarrierWave will remove the files
after storing them. However, unlike a local file, with object storage,
the file is still copied, so setting `move_to_store` to `true`
doesn't buy us anything.

To ensure files are cleaned up, we can just inherit from the
GitlabUploader implementation of `move_to_store`, which returns `true`
if it's a local file, `false` otherwise.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/60656

We've seen a significant performance penalty when using
`BatchLoader#__replace_with!`. This defines methods on the batch loader
that proxy to the 'real' object using send. The alternative is
`method_missing`, which is slower.  However, we've noticed that
`method_missing` can be faster if:

1. The objects being loaded have a large interface.
2. We don't call too many methods on the loaded object.

Avatar uploads meet both criteria above, so let's use the newly-released
feature in https://github.com/exAspArk/batch-loader/pull/45.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/60903