Use constant size generic parameter for random bytes generation

All uses of `get_random()` were in the form of:

  `&get_random(vec![0u8; SIZE])`

with `SIZE` being a constant.

Building a `Vec` is unnecessary for two reasons. First, it uses a
very short-lived dynamic memory allocation. Second, a `Vec` is a
resizable object, which is useless in those context when random
data have a fixed size and will only be read.

`get_random_bytes()` takes a constant as a generic parameter and
returns an array with the requested number of random bytes.

Stack safety analysis: the random bytes will be allocated on the
caller stack for a very short time (until the encoding function has
been called on the data). In some cases, the random bytes take
less room than the `Vec` did (a `Vec` is 24 bytes on a 64 bit
computer). The maximum used size is 180 bytes, which makes it
for 0.008% of the default stack size for a Rust thread (2MiB),
so this is a non-issue.

Also, most of the uses of those random bytes are to encode them
using an `Encoding`. The function `crypto::encode_random_bytes()`
generates random bytes and encode them with the provided
`Encoding`, leading to code deduplication.

`generate_id()` has also been converted to use a constant generic
parameter as well since the length of the requested String is always
a constant.

src/api/core/two_factor/authenticator.rs

@@ -34,7 +34,7 @@ async fn generate_authenticator(data: JsonUpcase<PasswordData>, headers: Headers
 
     let (enabled, key) = match twofactor {
         Some(tf) => (true, tf.data),
-        _ => (false, BASE32.encode(&crypto::get_random(vec![0u8; 20]))),
+        _ => (false, crypto::encode_random_bytes::<20>(BASE32)),
     };
 
     Ok(Json(json!({

src/api/core/two_factor/mod.rs

@@ -105,7 +105,7 @@ async fn recover(data: JsonUpcase<RecoverTwoFactor>, mut conn: DbConn) -> JsonRe
 
 async fn _generate_recover_code(user: &mut User, conn: &mut DbConn) {
     if user.totp_recover.is_none() {
-        let totp_recover = BASE32.encode(&crypto::get_random(vec![0u8; 20]));
+        let totp_recover = crypto::encode_random_bytes::<20>(BASE32);
         user.totp_recover = Some(totp_recover);
         user.save(conn).await.ok();
     }

src/api/notifications.rs

@@ -56,7 +56,7 @@ fn negotiate(_headers: Headers) -> Json<JsonValue> {
     use crate::crypto;
     use data_encoding::BASE64URL;
 
-    let conn_id = BASE64URL.encode(&crypto::get_random(vec![0u8; 16]));
+    let conn_id = crypto::encode_random_bytes::<16>(BASE64URL);
     let mut available_transports: Vec<JsonValue> = Vec::new();
 
     if CONFIG.websocket_enabled() {