Fix vertical centering of spinner

src/plugins/chatview/styles/index.scss

@@ -49,9 +49,6 @@
             height: var(--overlayed-chat-height);
             min-height: calc(var(--overlayed-chat-height) / 2);
         }
-        .chat-head {
-            min-height: var(--overlayed-chat-head-height);
-        }
         .minimized-chats-flyout .chat-head {
             cursor: default;
         }

src/templates/styles/spinner.scss

@@ -1,3 +1,10 @@
+converse-spinner {
+    &.vertically-centered {
+        height: 100%;
+        margin-top: 30%;
+    }
+}
+
 .spinner-grow {
     .vertically-centered > & {
         margin-top: 30%;