Add font-size-monospace variable

app/assets/stylesheets/framework/filters.scss

@@ -453,7 +453,7 @@
 .search-token-target-branch {
   .value {
     font-family: $monospace-font;
-    font-size: 13px;
+    font-size: $gl-font-size-monospace;
   }
 }
 

app/assets/stylesheets/framework/typography.scss

@@ -604,7 +604,7 @@ pre {
   display: block;
   padding: $gl-padding-8 $input-horizontal-padding;
   margin: 0 0 $gl-padding-8;
-  font-size: 13px;
+  font-size: $gl-font-size-monospace;
   word-break: break-all;
   word-wrap: break-word;
   color: $gl-text-color;
@@ -646,7 +646,7 @@ code {
  */
 textarea.js-gfm-input {
   font-family: $monospace-font;
-  font-size: 13px;
+  font-size: $gl-font-size-monospace;
 }
 
 .strikethrough {

app/assets/stylesheets/framework/variables.scss

@@ -380,6 +380,7 @@ $gl-font-size-14: 14px;
 $gl-font-size-16: 16px;
 $gl-font-size-28: 28px;
 $gl-font-size-42: 42px;
+$gl-font-size-monospace: 13px;
 
 $type-scale: (
   1: 12px,

app/assets/stylesheets/pages/note_form.scss

@@ -305,7 +305,7 @@ table {
 
   .reply-placeholder-text-field {
     font-family: $monospace-font;
-    font-size: 13px;
+    font-size: $gl-font-size-monospace;
     border-radius: $gl-border-radius-base;
     width: 100%;
     resize: none;