Add auto save functionality

Fixes #527

This adds auto saving to gollum using local storage.

lib/gollum/public/gollum/javascript/editor/gollum.editor.js

@@ -51,10 +51,37 @@
       }).insertAfter(textarea);
       textarea.css('display', 'none');
 
+      // Create a div for the autosave status message
+      var autoSaveMessage = document.createElement('div');
+      $(autoSaveMessage).addClass('div position-fixed bottom-0 left-0');
+      $('body').append(autoSaveMessage);
+
+      var autoSaveTimer = null;
+
       // NOTE: This requires the page to have only one 'gollum-editor-body'.
       var editor = ace.edit(editDiv[0], {rtlText: true});
       window.ace_editor = editor;
 
+      // Check to see if we have any autosaved text and show a message to
+      // restore it if present.
+      var storageKey = 'gollum_autorecover_' + window.location;
+      var savedText = localStorage.getItem(storageKey);
+
+      if (savedText) {
+        var savedMsg = document.createElement('div');
+        var restoreButton = document.createElement('button');
+        $(restoreButton).addClass('btn btn-sm primary flash-action')
+          .text('Restore Text')
+          .click(function() {
+            editor.getSession().setValue(savedText);
+            $(savedMsg).remove()
+          });
+        $(savedMsg).addClass('flash flash-full')
+          .text('Autosaved text is available. Click the button to restore it.')
+          .append(restoreButton);
+        $('body').prepend(savedMsg);
+      }
+
       editor.setTheme("ace/theme/tomorrow");
       editor.setKeyboardHandler();
       editor.renderer.setShowGutter(false);
@@ -64,6 +91,20 @@
 
       editor.getSession().on('change', function(){
         textarea.val(editor.getSession().getValue());
+
+        // Autosave
+        if (autoSaveTimer) {
+          // Reset the timer because we just changed the text
+          clearTimeout(autoSaveTimer);
+        }
+
+        $(autoSaveMessage).text('Saving...');
+
+        // Wait 2 seconds, then actualy save the text to local storage
+        autoSaveTimer = setTimeout(function() {
+          localStorage.setItem(storageKey, editor.getSession().getValue());
+          $(autoSaveMessage).text('Saved recovery text');
+        }, 2000);
       });
 
       if (isRTL(editor.getSession().getLine(0))) {
@@ -114,6 +155,12 @@
       editor.focus();
     });
 
+    // Remove any autosaved text when we hit save or cancel
+    $("#gollum-editor-submit, #gollum-editor-cancel").click(function() {
+      var storageKey = 'gollum_autorecover_' + window.location;
+      localStorage.removeItem(storageKey);
+    });
+
     debug('GollumEditor loading');
 
     if ( EditorHas.baseEditorMarkup() ) {