Fix remaining linting errors, from enabling the prefer-const ESLint rule in the web/ directory

This covers the handful of cases that the `--fix` command couldn't deal with, and the changes aren't just fixing the linting errors but attempt to slightly improve the relevant code.

web/app.js

@@ -1182,8 +1182,7 @@ const PDFViewerApplication = {
       if (!labels || AppOptions.get("disablePageLabels")) {
         return;
       }
-      let i = 0,
-        numLabels = labels.length;
+      const numLabels = labels.length;
       if (numLabels !== this.pagesCount) {
         console.error(
           "The number of Page Labels does not match " +
@@ -1191,6 +1190,7 @@ const PDFViewerApplication = {
         );
         return;
       }
+      let i = 0;
       // Ignore page labels that correspond to standard page numbering.
       while (i < numLabels && labels[i] === (i + 1).toString()) {
         i++;
@@ -2004,10 +2004,9 @@ function webViewerTextLayerRendered(evt) {
   }
 }
 
-function webViewerPageMode(evt) {
+function webViewerPageMode({ mode }) {
   // Handle the 'pagemode' hash parameter, see also `PDFLinkService_setHash`.
-  let mode = evt.mode,
-    view;
+  let view;
   switch (mode) {
     case "thumbs":
       view = SidebarView.THUMBS;

web/pdf_find_bar.js

@@ -153,8 +153,8 @@ class PDFFindBar {
     if (!this.findResultsCount) {
       return; // No UI control is provided.
     }
-    let matchesCountMsg = "",
-      limit = MATCHES_COUNT_LIMIT;
+    const limit = MATCHES_COUNT_LIMIT;
+    let matchesCountMsg = "";
 
     if (total > 0) {
       if (total > limit) {

web/pdf_link_service.js

@@ -108,8 +108,8 @@ class PDFLinkService {
   navigateTo(dest) {
     const goToDestination = ({ namedDest, explicitDest }) => {
       // Dest array looks like that: <page-ref> </XYZ|/FitXXX> <args..>
-      let destRef = explicitDest[0],
-        pageNumber;
+      const destRef = explicitDest[0];
+      let pageNumber;
 
       if (destRef instanceof Object) {
         pageNumber = this._cachedPageNumber(destRef);
@@ -405,8 +405,7 @@ function isValidExplicitDestination(dest) {
   if (!Array.isArray(dest)) {
     return false;
   }
-  let destLength = dest.length,
-    allowNull = true;
+  const destLength = dest.length;
   if (destLength < 2) {
     return false;
   }
@@ -425,6 +424,7 @@ function isValidExplicitDestination(dest) {
   if (!(typeof zoom === "object" && typeof zoom.name === "string")) {
     return false;
   }
+  let allowNull = true;
   switch (zoom.name) {
     case "XYZ":
       if (destLength !== 5) {