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.

sass/includes/base.sass

@@ -54,7 +54,6 @@ details
   border-radius: 8px
 
   summary
-    text-decoration: underline
     cursor: pointer
     line-height: 1
     margin-bottom: 0

sass/includes/subscriptions-page.sass

@@ -33,3 +33,35 @@
   .name
     font-size: 22px
     color: c.$fg-main
+
+
+#watched-videos-display
+  position: relative
+  left: 10px
+  display: block
+  z-index: 1
+  height: 42px
+  margin: 0
+
+.watched-videos-display-container
+  position: relative
+  display: grid // why does the default not work???
+  top: -42px
+  background: c.$bg-accent-x
+  line-height: 1
+  border-radius: 8px
+  margin-bottom: -18px
+
+  .watched-videos-display-label
+    padding: 12px 0px 12px 32px
+    cursor: pointer
+
+#watched-videos-display:checked ~ .video-list-item--watched
+  display: none
+
+.video-list-item--watched
+  background: c.$bg-darker
+  padding: 8px 8px 0px
+
+  .thumbnail .image
+    opacity: 0.4