auspice-client: Use import for image file

The new version of Auspice (which uses Webpack v5) uses a built-in Asset
Module (asset/resource) to handle files that were previously handled
with file-loader¹.

file-loader in Webpack v4 is documented to handle both import and
require()². Based on docs which suggest that asset/resource directly
replaces file-loader³, I'd expect it to also support require(). It seems
like that is not the case, so this is a workaround.

¹ nextstrain/auspice@7892c59
² https://v4.webpack.js.org/loaders/file-loader/
³ https://webpack.js.org/guides/asset-modules/

auspice-client/customisations/navbar.js

@@ -2,7 +2,7 @@
 import React from "react"; // eslint-disable-line
 import LanguageSelector from "./languageSelector.js";
 
-const logoPNG = require("./nextstrain-logo-small.png");
+import logoPNG from "./nextstrain-logo-small.png";
 
 /* This code is straight from the auspice repo.
  * There are theme props available if one used styled components here