Add explanation to featured hashtags page and profile (#11586)

app/javascript/styles/mastodon/widgets.scss

@@ -109,6 +109,15 @@
   box-shadow: 0 0 15px rgba($base-shadow-color, 0.2);
 }
 
+.placeholder-widget {
+  padding: 16px;
+  border-radius: 4px;
+  border: 2px dashed $dark-text-color;
+  text-align: center;
+  color: $darker-text-color;
+  margin-bottom: 10px;
+}
+
 .contact-widget,
 .landing-page__information.contact-widget {
   box-sizing: border-box;
@@ -526,6 +535,12 @@ $fluid-breakpoint: $maximum-width + 20px;
   a {
     font-size: 14px;
     line-height: 20px;
+  }
+}
+
+.notice-widget,
+.placeholder-widget {
+  a {
     text-decoration: none;
     font-weight: 500;
     color: $ui-highlight-color;