js: Automatically convert var to let and const in most files.

This commit was originally automatically generated using `tools/lint
--only=eslint --fix`.  It was then modified by tabbott to contain only
changes to a set of files that are unlikely to result in significant
merge conflicts with any open pull request, excluding about 20 files.
His plan is to merge the remaining changes with more precise care,
potentially involving merging parts of conflicting pull requests
before running the `eslint --fix` operation.

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

static/js/lightbox_canvas.js

@@ -1,4 +1,4 @@
-var events = {
+const events = {
     documentMouseup: [],
     windowResize: [],
 };
@@ -44,11 +44,11 @@ const funcs = {
     // this is a function given a canvas that attaches all of the events
     // required to pan and zoom.
     attachEvents: function (canvas, context, meta) {
-        var mousedown = false;
+        let mousedown = false;
 
         // wheelEvent.deltaMode is a value that describes what the unit is
         // for the `deltaX`, `deltaY`, and `deltaZ` properties.
-        var DELTA_MODE = {
+        const DELTA_MODE = {
             PIXEL: 0,
             LINE: 1,
             PAGE: 2,
@@ -56,13 +56,13 @@ const funcs = {
 
         // give object structure in `mousedown`, because its props are only
         // ever set once `mousedown` + `mousemove` is triggered.
-        var lastPosition = {};
+        let lastPosition = {};
 
         // in browsers such as Safari, the `e.movementX` and `e.movementY`
         // props don't exist, so we need to create them as a difference of
         // where the last `layerX` and `layerY` movements since the last
         // `mousemove` event in this `mousedown` event were registered.
-        var polyfillMouseMovement = function (e) {
+        const polyfillMouseMovement = function (e) {
             e.movementX = e.layerX - lastPosition.x || 0;
             e.movementY = e.layerY - lastPosition.y || 0;
 
@@ -80,7 +80,7 @@ const funcs = {
             e.preventDefault();
 
             // this is to reverese scrolling directions for the image.
-            var delta = meta.direction * e.deltaY;
+            let delta = meta.direction * e.deltaY;
 
             if (e.deltaMode === DELTA_MODE.LINE) {
                 // the vertical height in pixels of an approximate line.
@@ -99,7 +99,7 @@ const funcs = {
             // delta = 8
             // normalizedDelta = delta * (1 / 20) * 1 = 0.4
             // zoom = zoom * (0.4 / 100) + 1
-            var zoom = meta.zoom * (
+            const zoom = meta.zoom * (
                 meta.speed * meta.internalSpeedMultiplier * delta / 100 + 1
             );
 
@@ -122,7 +122,7 @@ const funcs = {
                 polyfillMouseMovement(e);
                 // find the percent of movement relative to the canvas width
                 // since e.movementX, e.movementY are in px.
-                var percentMovement = {
+                const percentMovement = {
                     x: e.movementX / canvas.width,
                     y: e.movementY / canvas.height,
                 };
@@ -205,15 +205,15 @@ const funcs = {
         meta.coords.y = Math.max(1 / (meta.zoom * 2), meta.coords.y);
         meta.coords.y = Math.min(1 - 1 / (meta.zoom * 2), meta.coords.y);
 
-        var c = {
+        const c = {
             x: meta.coords.x - 1,
             y: meta.coords.y - 1,
         };
 
-        var x = meta.zoom * c.x * canvas.width + canvas.width / 2;
-        var y = meta.zoom * c.y * canvas.height + canvas.height / 2;
-        var w = canvas.width * meta.zoom;
-        var h = canvas.height * meta.zoom;
+        const x = meta.zoom * c.x * canvas.width + canvas.width / 2;
+        const y = meta.zoom * c.y * canvas.height + canvas.height / 2;
+        const w = canvas.width * meta.zoom;
+        const h = canvas.height * meta.zoom;
 
         canvas.width = canvas.width;
         context.imageSmoothingEnabled = false;
@@ -233,7 +233,7 @@ const funcs = {
             meta.onresize(canvas);
         }
 
-        var parent = {
+        const parent = {
             width: canvas.parentNode.clientWidth,
             height: canvas.parentNode.clientHeight,
         };
@@ -258,8 +258,8 @@ const funcs = {
 };
 
 // a class w/ prototype to create a new `LightboxCanvas` instance.
-var LightboxCanvas = function (el) {
-    var self = this;
+const LightboxCanvas = function (el) {
+    const self = this;
 
     this.meta = {
         direction: -1,