REFACTOR: Do not rely on module order for discourse-markdown loading

Before this patch, discourse-markdown depended on the modules in its
bundle being defined in a specific order or it wouldn't load properly.

Now, any file in the bundle can export a `priority` const (default 0)
and files will be loaded in order of ascending priority instead. This
allows us to use a bundle packaged in any order we want.

app/assets/javascripts/pretty-text/engines/discourse-markdown/resize-controls.js

@@ -61,6 +61,9 @@ function buildScaleButton(selectedScale, scale) {
   );
 }
 
+// We need this to load after `upload-protocol` which is priority 0
+export const priority = 1;
+
 export function setup(helper) {
   const opts = helper.getOptions();
   if (opts.previewing) {