add missing <select> styles

src/stylesheets/index.scss

@@ -1,6 +1,7 @@
 @import "primer-base/index";
 @import "primer-buttons/lib/button";
 @import "primer-forms/lib/form-control";
+@import "primer-forms/lib/form-select";
 @import "primer-forms/lib/form-group";
 @import "primer-forms/lib/input-group";
 @import "primer-markdown/index";

src/stylesheets/timeline-comment.scss

@@ -47,6 +47,7 @@
     padding: $spacer-2;
 
     textarea {
+      appearance: none;
       display: block;
       max-height: 550px;
       resize: vertical;

src/stylesheets/util.scss

@@ -12,7 +12,3 @@
 .markdown-body .highlight-source-js > pre {
   -webkit-overflow-scrolling: touch;
 }
-
-input, select, textarea {
-  appearance: none;
-}