[AB-xxx] fixing incorrectly rendering multiple attachments for link embeds and starboard posts

abstracto-modules/link-embed/src/main/resources/default/listener/message_embed_embed.ftl

@@ -75,9 +75,9 @@
                     <#items as attachment>
                     {
                         "url": "${attachment.proxyUrl}"
-                    }</#items>
+                    }<#sep>,</#items>
                 ]
-            }<#sep>,</#list>
+            }</#list>
         </#if>
         <#if hasContent==false>
             {

abstracto-modules/starboard/src/main/resources/default/listener/starboard_post_embed.ftl

@@ -89,9 +89,9 @@
                     <#items as attachment>
                         {
                             "url": "${attachment.proxyUrl}"
-                        }</#items>
+                        }<#sep>,</#items>
                     ]
-                }<#sep>,</#list>
+                }</#list>
             </#if>
             <#if hasContent==false>
                 {