Add ability to reorder uploaded media before posting in web UI (#28456)

app/javascript/mastodon/actions/compose.js

@@ -75,6 +75,7 @@ export const INIT_MEDIA_EDIT_MODAL = 'INIT_MEDIA_EDIT_MODAL';
 
 export const COMPOSE_CHANGE_MEDIA_DESCRIPTION = 'COMPOSE_CHANGE_MEDIA_DESCRIPTION';
 export const COMPOSE_CHANGE_MEDIA_FOCUS       = 'COMPOSE_CHANGE_MEDIA_FOCUS';
+export const COMPOSE_CHANGE_MEDIA_ORDER       = 'COMPOSE_CHANGE_MEDIA_ORDER';
 
 export const COMPOSE_SET_STATUS = 'COMPOSE_SET_STATUS';
 export const COMPOSE_FOCUS = 'COMPOSE_FOCUS';
@@ -809,3 +810,9 @@ export function changePollSettings(expiresIn, isMultiple) {
     isMultiple,
   };
 }
+
+export const changeMediaOrder = (a, b) => ({
+  type: COMPOSE_CHANGE_MEDIA_ORDER,
+  a,
+  b,
+});