Let user set default thumbnail universally or per category

assets/javascripts/discourse/connectors/category-custom-settings/enable-topic-list-thumbnails.hbs

@@ -25,3 +25,8 @@
     {{i18n 'category.list_category_badge_move'}}
   </label>
 </section>
+
+<section class='field'>
+  <label>{{i18n 'category.list_default_thumbnail'}}</label>
+  {{text-field value=category.custom_fields.list_default_thumbnail placeholderKey="category.list_default_thumbnail_placeholder"}}
+</section>

assets/javascripts/discourse/connectors/topic-list-tags/topic-list-preview.raw.hbs

@@ -2,7 +2,7 @@
   {{#if context.showThumbnail}}
     <div class="topic-thumbnail">
       <a href='{{context.topic.url}}'>
-        {{preview-unbound context.topic.thumbnails}}
+        {{preview-unbound context.thumbnails}}
       </a>
     </div>
   {{/if}}

assets/javascripts/discourse/initializers/preview-edits.js.es6

@@ -161,8 +161,19 @@ export default {
 
       @computed()
       showThumbnail() {
-        return this.get('topic.thumbnails') && (Discourse.SiteSettings.topic_list_thumbnails ||
-                                               (this.get('category') && this.get('category.list_thumbnails')))
+        return this.get('thumbnails') && (Discourse.SiteSettings.topic_list_thumbnails ||
+               (this.get('category') && this.get('category.list_thumbnails')))
+      },
+
+      @computed()
+      defaultThumbnail(){
+        let defaultThumbnail = this.get('category.list_default_thumbnail') || Discourse.SiteSettings.topic_list_default_thumbnail
+        return defaultThumbnail ? defaultThumbnail : false
+      },
+
+      @computed()
+      thumbnails(){
+        return this.get('topic.thumbnails') || this.get('defaultThumbnail')
       },
 
       @computed()

assets/javascripts/lib/render-preview.js.es6

@@ -1,5 +1,5 @@
 var renderUnboundPreview = function(thumbnails) {
-  var previewUrl = window.devicePixelRatio >= 2 ? thumbnails.retina : thumbnails.normal
+  let previewUrl = thumbnails.retina ? (window.devicePixelRatio >= 2 ? thumbnails.retina : thumbnails.normal) : thumbnails
   return '<img class="thumbnail" src="' + previewUrl + '" onError="this.onerror=null;this.src=\'\';this.className=\'no-thumbnail\';" />';
 };
 

config/locales/client.en.yml

@@ -5,3 +5,5 @@ en:
       list_excerpts: "Show topic excerpts in topic list items from this category."
       list_actions: "Show like and bookmark buttons in topic list items from this category."
       list_category_badge_move: "Show category badge under title in topic list from this category."
+      list_default_thumbnail: "Image URL for thumbnail shown if topic in this category has no preview thumbnail set."
+      list_default_thumbnail_placeholder: "Default thumbnail URL"

config/locales/server.en.yml

@@ -6,3 +6,4 @@ en:
     topic_list_category_badge_move: "Show the category badge under the title of all topic list items."
     topic_list_show_like_on_current_users_posts: "Show an inactive like button in topic list items previewing a post created by the current user (non-default Discourse behavior)."
     topic_list_excerpt_length: "Character length of topic excerpts."
+    topic_list_default_thumbnail: "Image URL for thumbnail shown if topic has no preview thumbnail set."

config/settings.yml

@@ -17,3 +17,6 @@ plugins:
   topic_list_excerpt_length:
     default: 200
     client: true
+  topic_list_default_thumbnail:
+    default: ''
+    client: true

plugin.rb

@@ -197,4 +197,5 @@ def topic_post_can_unlike
   add_to_serializer(:basic_category, :list_thumbnails) {object.custom_fields["list_thumbnails"]}
   add_to_serializer(:basic_category, :list_actions) {object.custom_fields["list_actions"]}
   add_to_serializer(:basic_category, :list_category_badge_move) {object.custom_fields["list_category_badge_move"]}
+  add_to_serializer(:basic_category, :list_default_thumbnail) {object.custom_fields["list_default_thumbnail"]}
 end