Implement watched videos

Watched videos on your subscriptions feed will be darkened, and the
option to hide all of them has been added.

This only takes effect if you have enabled saving watched videos on the
server in the settings menu - default is off.

pug/includes/video-list-item.pug

@@ -1,19 +1,20 @@
-mixin video_list_item(video, instanceOrigin)
-  - let link = `/watch?v=${video.videoId}`
-  a(href=link tabindex="-1").thumbnail
-    img(src=`/vi/${video.videoId}/mqdefault.jpg` width=320 height=180 alt="").image
-    if video.second__lengthText != undefined
-      span.duration= video.second__lengthText
-  .info
-    div.title: a(href=link).title-link= video.title
-    div.author-line
-      a(href=`/channel/${video.authorId}`).author= video.author
-      - const views = video.viewCountText || video.second__viewCountText
-      if views
-        = ` • `
-        span.views= views
-      if video.publishedText
-        = ` • `
-        span.published= video.publishedText
-    if video.descriptionHtml
-      div.description!= video.descriptionHtml
+mixin video_list_item(className, video, instanceOrigin)
+  div(class={[className]: true, "video-list-item--watched": video.watched})
+    - let link = `/watch?v=${video.videoId}`
+    a(href=link tabindex="-1").thumbnail
+      img(src=`/vi/${video.videoId}/mqdefault.jpg` width=320 height=180 alt="").image
+      if video.second__lengthText != undefined
+        span.duration= video.second__lengthText
+    .info
+      div.title: a(href=link).title-link= video.title
+      div.author-line
+        a(href=`/channel/${video.authorId}`).author= video.author
+        - const views = video.viewCountText || video.second__viewCountText
+        if views
+          = ` • `
+          span.views= views
+        if video.publishedText
+          = ` • `
+          span.published= video.publishedText
+      if video.descriptionHtml
+        div.description!= video.descriptionHtml