js: Convert static/js/lightbox_canvas.js to ES6 module.

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

.eslintrc.json

@@ -131,7 +131,6 @@
                 "$": false,
                 "FetchStatus": false,
                 "Filter": false,
-                "LightboxCanvas": false,
                 "ListCursor": false,
                 "MessageListData": false,
                 "MessageListView": false,

static/js/bundles/app.js

@@ -17,7 +17,6 @@ import "../i18n";
 import "../loading";
 import "../vdom";
 import "../keydown_util";
-import "../lightbox_canvas";
 import "../rtl";
 import "../rendered_markdown";
 import "../fold_dict";

static/js/global.d.ts

@@ -5,7 +5,6 @@
 
 declare let FetchStatus: any;
 declare let Filter: any;
-declare let LightboxCanvas: any;
 declare let ListCursor: any;
 declare let MessageListData: any;
 declare let MessageListView: any;

static/js/lightbox.js

@@ -1,5 +1,6 @@
 "use strict";
 
+const {LightboxCanvas} = require("./lightbox_canvas");
 const people = require("./people");
 
 let is_open = false;

static/js/lightbox_canvas.js

@@ -1,5 +1,3 @@
-"use strict";
-
 const funcs = {
     setZoom(meta, zoom) {
         // condition to handle zooming event by zoom hotkeys
@@ -206,7 +204,7 @@ const funcs = {
     },
 };
 
-class LightboxCanvas {
+export class LightboxCanvas {
     meta = {
         direction: -1,
         zoom: 1,
@@ -270,6 +268,3 @@ class LightboxCanvas {
         this.meta.resize_handler = callback;
     }
 }
-
-module.exports = LightboxCanvas;
-window.LightboxCanvas = LightboxCanvas;