Fix: Images cannot show when projects' path was changed

CHANGELOG

app/models/namespace.rb

app/models/project.rb

app/services/projects/transfer_service.rb

app/uploaders/file_uploader.rb

features/steps/admin/projects.rb

lib/gitlab/markdown.rb

lib/gitlab/markdown/upload_link_filter.rb

lib/gitlab/uploads_transfer.rb

public/uploads/.gitkeep

spec/controllers/projects/uploads_controller_spec.rb

spec/lib/gitlab/email/attachment_uploader_spec.rb

spec/lib/gitlab/markdown/upload_link_filter_spec.rb

spec/lib/gitlab/uploads_transfer_spec.rb

spec/services/projects/download_service_spec.rb

spec/services/projects/transfer_service_spec.rb

spec/services/projects/upload_service_spec.rb