js: Use unminified KaTeX (and minifiy it with everything else).

Since our Webpack config passes pre-minified JS files to
script-loader, they can’t be used as modules.  Use the normal
unminified version, letting Webpack minify it and give us source maps.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

static/shared/js/fenced_code.js

@@ -1,4 +1,4 @@
-import katex from "katex/dist/katex.min.js";
+import katex from "katex";
 import _ from "lodash";
 
 // Parsing routine that can be dropped in to message parsing