Fix auto-reloading of multipart.rb

During development, it's quite common to see the error:

```
A copy of Gitlab::Middleware::Multipart has been removed from the module
tree but is still active!
```

Prefixing the uploaders with :: should prevent this from happening. See
https://stackoverflow.com/a/29710188/1992201 for more details.

lib/gitlab/middleware/multipart.rb

@@ -123,9 +123,9 @@ module Gitlab
         def allowed_paths
           [
             ::FileUploader.root,
-            Gitlab.config.uploads.storage_path,
-            JobArtifactUploader.workhorse_upload_path,
-            LfsObjectUploader.workhorse_upload_path,
+            ::Gitlab.config.uploads.storage_path,
+            ::JobArtifactUploader.workhorse_upload_path,
+            ::LfsObjectUploader.workhorse_upload_path,
             File.join(Rails.root, 'public/uploads/tmp')
           ] + package_allowed_paths
         end
@@ -139,9 +139,9 @@ module Gitlab
         encoded_message = env.delete(RACK_ENV_KEY)
         return @app.call(env) if encoded_message.blank?
 
-        message = Gitlab::Workhorse.decode_jwt(encoded_message)[0]
+        message = ::Gitlab::Workhorse.decode_jwt(encoded_message)[0]
 
-        Handler.new(env, message).with_open_files do
+        ::Gitlab::Middleware::Multipart::Handler.new(env, message).with_open_files do
           @app.call(env)
         end
       rescue UploadedFile::InvalidPathError => e