www: Add CSP to support copy button

Author: BeniCheni

www/index.html

@@ -5,7 +5,7 @@
   <title>rustup.rs - The Rust toolchain installer</title>
   <meta name="keywords" content="Rust, Rust programming language, rustlang, rust-lang, Mozilla Rust, rustup">
   <meta name="description" content="The Rust toolchain installer">
-
+  <meta http-equiv="Content-Security-Policy" content="default-src 'self'; script-src 'self' data: https://rustup.rs 'unsafe-inline'; img-src * 'self' data: https://www.rust-lang.org/logos">
   <link rel="stylesheet" href="normalize.css">
   <link rel="stylesheet" href="rustup.css">