Reduce media ID length from 256 to 32

Most common filesystems limit paths to 255 bytes.
This change brings down the media ID length to be similar to
Synapse servers (25), and makes it possible for clients to
download media with the ID included in the filename.

src/client_server/media.rs

@@ -9,7 +9,7 @@ use ruma::api::client::{
 use rocket::{get, post};
 use std::convert::TryInto;
 
-const MXC_LENGTH: usize = 256;
+const MXC_LENGTH: usize = 32;
 
 #[cfg_attr(feature = "conduit_bin", get("/_matrix/media/r0/config"))]
 pub fn get_media_config_route(