message: fix new clippy error
diff: use crate::config::Config;
  use iron::{AfterMiddleware, BeforeMiddleware, IronResult, Request, Response};
  +use std::fmt::Write;
  pub(super) struct Csp {
  nonce: String,
  @@ -67,7 +68,9 @@ impl Csp {
  // We can't just allow 'self' here, as users can upload arbitrary .js files as part of
  // their documentation and 'self' would allow their execution. Instead, every allowed
  // script must include the random nonce in it, which an attacker is not able to guess.
  - result.push_str(&format!("; script-src 'nonce-{}'", self.nonce));
  + //
  + // This `.unwrap` is safe since the `Write` impl on str can never fail.
  + write!(result, "; script-src 'nonce-{}'", self.nonce).unwrap();
  }
  fn render_svg(&self, result: &mut String) {
  