Prevent tags from overlapping discussion titles

This gives the tag labels a max-width in the discussion list, and overflows them with a gradient to transparency. (Unfortunately this only works in WebKit for now.) When hovering over them, they slide out to show more.

Fixes flarum/core#702

extensions/tags/less/forum/extension.less

@@ -32,10 +32,24 @@
 }
 @media @tablet-up {
   .IndexPage, .UserPage {
+    .DiscussionListItem-title {
+      margin-right: 155px;
+    }
     .DiscussionListItem-info > .item-tags {
-      float: right;
-      margin-top: -12px;
       margin-right: 0;
+      position: absolute;
+      right: 80px;
+      top: 23px;
+      max-width: 150px;
+      white-space: nowrap;
+      overflow: hidden;
+      transition: max-width 0.2s ease-in-out, -webkit-mask-image 0.2s;
+      -webkit-mask-image: linear-gradient(to right, rgba(0,0,0,1) 140px, rgba(0,0,0,0) 150px);
+
+      &:hover {
+        max-width: 400px;
+        -webkit-mask-image: none;
+      }
     }
   }
 }