Fix tags autocomplete dropdown CSS

7fff01e changed the annotation cards from <article> to <annotation>.
This seems to break the CSS of the tags autocomplete dropdown
(particularly: the width is too short) unless the <annotation> has
display: block;

Fixes #3007.

h/static/styles/annotation.scss

@@ -17,6 +17,7 @@ $annotation-card-left-padding: 10px;
 }
 
 .annotation {
+  display: block;
   font-family: $sans-font-family;
   font-weight: 300;
   position: relative;