Use localStorage rather than sessionStorage for storing user prefs

- Also create some helper functions

InvenTree/InvenTree/static/script/inventree/inventree.js

@@ -136,4 +136,47 @@ function imageHoverIcon(url) {
         `;
 
     return html;
+}
+
+function inventreeSave(name, value) {
+    /*
+     * Save a key:value pair to local storage
+     */
+
+    var key = "inventree-" + name;
+    localStorage.setItem(key, value);
+}
+
+function inventreeLoad(name) {
+    /* 
+     * Retrieve a key:value pair from local storage
+     */
+
+    var key = "inventree-" + name;
+
+    return localStorage.getItem(key);
+}
+
+function inventreeLoadInt(name) {
+    /*
+     * Retrieve a value from local storage, and attempt to cast to integer
+     */
+
+    var data = inventreeLoad(name);
+
+    return parseInt(data, 10);
+}
+
+function inventreeLoadFloat(name) {
+
+    var data = inventreeLoad(name);
+
+    return parseFloat(data);
+}
+
+function inventreeDel(name) {
+
+    var key = 'inventree-' + name;
+
+    localStorage.removeItem(key);
 }

InvenTree/part/templates/part/category.html

@@ -67,16 +67,16 @@
 {% block js_ready %}
 {{ block.super }}
 
-    if (sessionStorage.getItem("inventree-show-part-categories")) {
+    if (inventreeLoadInt("show-part-cats") == 1) {
         $("#collapse-item-categories").collapse('show');
     }
 
     $("#collapse-item-categories").on('shown.bs.collapse', function() {
-        sessionStorage.setItem('inventree-show-part-categories', 1);
+        inventreeSave('show-part-cats', 1);
     });
     
     $("#collapse-item-categories").on('hidden.bs.collapse', function() {
-        sessionStorage.removeItem('inventree-show-part-categories');
+        inventreeDel('show-part-cats');
     });
 
     $("#cat-create").click(function() {

InvenTree/stock/templates/stock/location.html

@@ -55,16 +55,16 @@
 {% block js_ready %}
 {{ block.super }}
 
-    if (sessionStorage.getItem("inventree-show-part-locations")) {
+    if (inventreeLoadInt("show-part-locs") == 1) {
         $("#collapse-item-locations").collapse('show');
     }
 
     $("#collapse-item-locations").on('shown.bs.collapse', function() {
-        sessionStorage.setItem('inventree-show-part-locations', 1);
+        inventreeSave('show-part-locs', 1);
     });
 
     $("#collapse-item-locations").on('hidden.bs.collapse', function() {
-        sessionStorage.removeItem('inventree-show-part-locations');
+        inventreeDel('show-part-locs');
     });
 
     $("#stock-export").click(function() {