Fix poll options checkboxes/radio buttons not being vertically centered (mastodon#11833)

Author: ClearlyClaire

app/javascript/styles/mastodon/polls.scss

@@ -79,6 +79,9 @@
     top: -1px;
     border-radius: 50%;
     vertical-align: middle;
+    margin-top: auto;
+    margin-bottom: auto;
+    flex: 0 0 18px;
 
     &.checkbox {
       border-radius: 4px;