Reduce memory usage when uploading files by a factor of 11

[jyn514]

Addresses #658. A future PR could reduce usage by a further factor of 2 by removing unnecessary clone()s in the upload code. Done.

This uses the Rust mime_guess package instead libmagic.
Most work was already done in #600.
This just sets mime_guess to build unconditionally instead of only on Windows.

This improves the memory usage of docs.rs by over a factor of 5 when
uploading files in some cases. This can be tested locally like so:

Run minio in the background:

docker run -p 9000:9000 -e MINIO_ACCESS_KEY=password -e MINIO_SECRET_KEY=password minio/minio server /data

Navigate to localhost:9000/ and add a new bucket called rust-docs-rs.

Now in another terminal, run docs.rs. This assumes you already have the
CRATESFYI_DATABASE_URL set up locally as described in
developing without docker-compose:

 # set up a fairly large file
mkdir -p ignored/tmp
yes | head -n $((1024 * 1024 * 50)) > ignored/tmp/100MB

git checkout master
cargo build --release
RUST_LOG=cratesfyi,info S3_ENDPOINT=http://localhost:9000 AWS_ACCESS_KEY_ID=password AWS_SECRET_ACCESS_KEY=password valgrind --tool=massif --massif-out-file=libmagic.massif.out target/release/cratesfyi database add-directory ignored/tmp/
 # this will show ~1.5 GB used, depending on your system
ms_print libmagic.massif.out | less

 # now try without libmagic
git checkout druid
cargo build --release
RUST_LOG=cratesfyi,info S3_ENDPOINT=http://localhost:9000 AWS_ACCESS_KEY_ID=password AWS_SECRET_ACCESS_KEY=password valgrind --tool=massif --massif-out-file=no_libmagic.massif.out target/release/cratesfyi database add-directory ignored/tmp/
 # this will show ~250 MB used, depending on your system
ms_print no_libmagic.massif.out | less

cc @Zexbe

r? @Mark-Simulacrum

[Mark-Simulacrum]

I'd also like to check if we can avoid allocating a String for the mime type - it looks like internally it's a &'static str in the library.

[jyn514]

Note that this now moves from setting the content-type based on the contents to based on the file extension. I don't foresee this impacting us, but I'll run a build with the new library and make sure that everything looks about right in the browser.

[jyn514]

Everything looks fine on a local build.

[jyn514]

After the last commit, this now only uses 129 MB locally, for a reduction of over 11x.

[Mark-Simulacrum]

I'm happy to merge if you're ready, I think this is good to go.