storage: fix s3 batch delete with unicode filenames

[syphar]

The back cleanup of the s3 bucket finally showed the actual error we're getting: with certain unicode chars in filenames, the batch-delete fails. This fixes it.

The error is:

error running admin CLI: failed to delete files from s3: ["sources/luminance-gl/0.5.0/\u{1b}", "sources/luminance-gl/0.5.0/.cargo-ok", "sources/luminance-gl/0.5.0/.gitignore", "sources/luminance-gl/0.5.0/.travis.yml", "sources/luminance-gl/0.5.0/CHANGELOG.md", "sources/luminance-gl/0.5.0/Cargo.toml", "sources/luminance-gl/0.5.0/LICENSE", "sources/luminance-gl/0.5.0/README.md", "sources/luminance-gl/0.5.0/src/error.rs", "sources/luminance-gl/0.5.0/src/gl33/buffer.rs", "sources/luminance-gl/0.5.0/src/gl33/framebuffer.rs", "sources/luminance-gl/0.5.0/src/gl33/mod.rs", "sources/luminance-gl/0.5.0/src/gl33/pipeline.rs", "sources/luminance-gl/0.5.0/src/gl33/pixel.rs", "sources/luminance-gl/0.5.0/src/gl33/shader/mod.rs", "sources/luminance-gl/0.5.0/src/gl33/shader/program.rs", "sources/luminance-gl/0.5.0/src/gl33/shader/stage.rs", "sources/luminance-gl/0.5.0/src/gl33/shader/uniform.rs", "sources/luminance-gl/0.5.0/src/gl33/tessellation.rs", "sources/luminance-gl/0.5.0/src/gl33/texture.rs", "sources/luminance-gl/0.5.0/src/gl33/token.rs", "sources/luminance-gl/0.5.0/src/lib.rs"]

Caused by:
    0: service error
    1: unhandled error (MalformedXML)
    2: Error { code: "MalformedXML", message: "The XML you provided was not well-formed or did not validate against our published schema", s3_extended_request_id: "cgqB+l/AMGTBA621/hEC035ZfeX/PvdWyoQTrrst1L8+gZZvlU8IiyyZzl4ZdGAo0rFvyyd4M1A=", aws_request_id: "1GZ7TEFFEFPG6T34" }

Stack backtrace:
   0: <aws_smithy_runtime_api::client::result::SdkError<aws_sdk_s3::operation::delete_objects::DeleteObjectsError, aws_smithy_runtime_api::http::response::Response> as anyhow::context::ext::StdError>::
ext_context::<alloc::string::String>
   1: <futures_util::stream::try_stream::try_for_each::TryForEach<futures_util::stream::stream::map::Map<futures_util::stream::stream::chunks::Chunks<core::pin::Pin<alloc::boxed::Box<dyn futures_core::stream::Stream<Item = core::result::Result<alloc::string::String, anyhow::Error>> + core::marker::Send>>>, <docs_rs_storage::backends::s3::S3Backend as docs_rs_storage::backends::StorageBackendMethods>::delete_prefix::{closure#0}::{closure#0}>, <docs_rs_storage::backends::s3::S3Backend>::delete_batch_with_retry::{closure#0}, <docs_rs_storage::backends::s3::S3Backend as docs_rs_storage::backends::Sto
rageBackendMethods>::delete_prefix::{closure#0}::{closure#1}> as core::future::future::Future>::poll
   2: <docs_rs_storage::storage::non_blocking::AsyncStorage>::delete_prefix::{closure#0}::{closure#0}
   3: <tracing::instrument::Instrumented<<docs_rs_storage::storage::non_blocking::AsyncStorage>::delete_prefix::{closure#0}::{closure#0}> as core::future::future::Future>::poll
   4: docs_rs_admin::cleanup_s3::clean_prefix::<alloc::string::String>::{closure#0}
   5: <tracing::instrument::Instrumented<docs_rs_admin::cleanup_s3::cleanup_s3_bucket::{closure#0}::{closure#0}> as core::future::future::Future>::poll
   6: <docs_rs_admin::DatabaseSubcommand>::handle_args::{closure#0}
   7: <docs_rs_admin::CommandLine>::handle_args::{closure#0}
   8: <tokio::runtime::park::CachedParkThread>::block_on::<docs_rs_admin::main::{closure#0}>
   9: tokio::runtime::context::runtime::enter_runtime::<<tokio::runtime::scheduler::multi_thread::MultiThread>::block_on<docs_rs_admin::main::{closure#0}>::{closure#0}, core::result::Result<(), anyhow:
:Error>>
  10: <tokio::runtime::runtime::Runtime>::block_on::<docs_rs_admin::main::{closure#0}>
  11: docs_rs_admin::main
  12: std::sys::backtrace::__rust_begin_short_backtrace::<fn() -> core::result::Result<(), anyhow::Error>, core::result::Result<(), anyhow::Error>>
  13: std::rt::lang_start::<core::result::Result<(), anyhow::Error>>::{closure#0}
  14: std::rt::lang_start_internal
  15: main
  16: __libc_start_main
             at /build/glibc-Df2k3R/glibc-2.27/csu/../csu/libc-start.c:310:0
  17: _start