Ensure emoji render with regular font-weight (#11541) (#11545)

Emoji characters don't support bold attributes and may do weird things
like rendering monochrome when bolded.

web_src/less/_base.less

 .reaction {
     font-size: 1.5em;
     line-height: 1.2;
-    font-weight: 400;
+    font-style: normal !important;
+    font-weight: normal !important;
     vertical-align: middle;
-    font-style: normal;
 }
 
 #issue-title > .emoji {