cmd/anubis: allow setting key bytes in flag/envvar (#97)

* cmd/anubis: allow setting key bytes in flag/envvar

Docs are updated to generate a random key on load and when people press
the recycle button.

Signed-off-by: Xe Iaso <me@xeiaso.net>

* review feedback fixups

Signed-off-by: Xe Iaso <me@xeiaso.net>

* Update cmd/anubis/main.go

Signed-off-by: Xe Iaso <me@xeiaso.net>

* Apply suggestions from code review

Co-authored-by: Ryan Cao <70191398+ryanccn@users.noreply.github.com>
Signed-off-by: Xe Iaso <me@xeiaso.net>

---------

Signed-off-by: Xe Iaso <me@xeiaso.net>
Co-authored-by: Ryan Cao <70191398+ryanccn@users.noreply.github.com>

docs/src/components/RandomKey/index.tsx

@@ -0,0 +1,42 @@
+import { useState, useCallback } from "react";
+import Code from "@theme/CodeInline";
+import BrowserOnly from "@docusaurus/BrowserOnly";
+
+// https://www.xaymar.com/articles/2020/12/08/fastest-uint8array-to-hex-string-conversion-in-javascript/
+function toHex(buffer) {
+  return Array.prototype.map
+    .call(buffer, (x) => ("00" + x.toString(16)).slice(-2))
+    .join("");
+}
+
+export const genRandomKey = (): String => {
+  const array = new Uint8Array(32);
+  self.crypto.getRandomValues(array);
+  return toHex(array);
+};
+
+export default function RandomKey() {
+  return (
+    <BrowserOnly fallback={<div>Loading...</div>}>
+      {() => {
+        const [key, setKey] = useState<String>(genRandomKey());
+        const genRandomKeyCb = useCallback(() => {
+          setKey(genRandomKey());
+        });
+        return (
+          <span>
+            <Code>{key}</Code>
+            <span style={{ marginLeft: "0.25rem", marginRight: "0.25rem" }} />
+            <button
+              onClick={() => {
+                genRandomKeyCb();
+              }}
+            >
+              ♻️
+            </button>
+          </span>
+        );
+      }}
+    </BrowserOnly>
+  );
+}