[Glitch] Fix poll options checkboxes/radio buttons not being vertically centered

Port d41494226d to glitch-soc

Signed-off-by: Thibaut Girka <thib@sitedethib.com>

app/javascript/flavours/glitch/styles/polls.scss

@@ -86,6 +86,9 @@
     top: -1px;
     border-radius: 50%;
     vertical-align: middle;
+    margin-top: auto;
+    margin-bottom: auto;
+    flex: 0 0 18px;
 
     &.checkbox {
       border-radius: 4px;