js: Convert self-referential vars to const.

ESLint won’t convert these automatically because it can’t rule out a
behavior difference arising from an access to a self-referential var
before it’s initialized:

> var x = (f => f())(() => x);
undefined
> let y = (f => f())(() => y);
Thrown:
ReferenceError: Cannot access 'y' before initialization
    at repl:1:26
    at repl:1:15

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

frontend_tests/zjsunit/zjquery.js

@@ -140,7 +140,7 @@ exports.make_new_elem = function (selector, opts) {
     var classes = new Dict();
     var event_store = exports.make_event_store(selector);
 
-    var self = {
+    const self = {
         addClass: function (class_name) {
             classes.set(class_name, true);
             return self;

static/js/feedback_widget.js

@@ -26,7 +26,7 @@ var meta = {
     opened: false,
 };
 
-var animate = {
+const animate = {
     maybe_close: function () {
         if (!meta.opened) {
             return;

static/js/input_pill.js

@@ -44,7 +44,7 @@ exports.create = function (opts) {
     // a dictionary of internal functions. Some of these are exposed as well,
     // and nothing in here should be assumed to be private (due to the passing)
     // of the `this` arg in the `Function.prototype.bind` use in the prototype.
-    var funcs = {
+    const funcs = {
         // return the value of the contenteditable input form.
         value: function (input_elem) {
             return input_elem.innerText.trim();

static/js/lightbox_canvas.js

@@ -29,7 +29,7 @@ window.onload = function () {
     });
 };
 
-var funcs = {
+const funcs = {
     setZoom: function (meta, zoom) {
         // condition to handle zooming event by zoom hotkeys
         if (zoom === '+') {

static/js/list_render.js

@@ -56,7 +56,7 @@ exports.create = function ($container, list, opts) {
         opts.filter = {};
     }
 
-    var prototype = {
+    const prototype = {
         // Reads the provided list (in the scope directly above)
         // and renders the next block of messages automatically
         // into the specified contianer.

static/js/localstorage.js

@@ -1,4 +1,4 @@
-var ls = {
+const ls = {
     // parse JSON without throwing an error.
     parseJSON: function (str) {
         try {

static/js/message_edit.js

@@ -375,7 +375,7 @@ function edit_message(row, raw_content) {
         // Do this right away, rather than waiting for the timer to do its first update,
         // since otherwise there is a noticeable lag
         message_edit_countdown_timer.text(timer_text(seconds_left));
-        var countdown_timer = setInterval(function () {
+        const countdown_timer = setInterval(function () {
             seconds_left -= 1;
             if (seconds_left <= 0) {
                 clearInterval(countdown_timer);