Add linter for JS files with basic recommended rules:

- Add Github Action.
- Fix JS files to match rules; mostly `globals` and `exported`.

bookwyrm/static/js/check_all.js

@@ -1,4 +1,4 @@
-// Toggle all checkboxes.
+/* exported toggleAllCheckboxes */
 
 /**
  * Toggle all descendant checkboxes of a target.

bookwyrm/static/js/localstorage.js

@@ -1,3 +1,6 @@
+/* exported updateDisplay */
+/* globals addRemoveClass */
+
 // set javascript listeners
 function updateDisplay(e) {
     // used in set reading goal

bookwyrm/static/js/shared.js

@@ -1,3 +1,5 @@
+/* globals setDisplay TabGroup toggleAllCheckboxes updateDisplay */
+
 // set up javascript listeners
 window.onload = function() {
     // buttons that display or hide content
@@ -93,7 +95,7 @@ function toggleAction(e) {
 
     // show/hide container
     var container = document.getElementById('hide-' + targetId);
-    if (!!container) {
+    if (container) {
         addRemoveClass(container, 'hidden', pressed);
     }
 

bookwyrm/static/js/tabs.js

@@ -1,3 +1,5 @@
+/* exported TabGroup */
+
 /*
 * The content below is licensed according to the W3C Software License at
 * https://www.w3.org/Consortium/Legal/2015/copyright-software-and-document