Fix small button line height

**Why?**
Commit 75496059 accidentally changed line height for all small buttons.
This fixes that by introducing some `btn-xs` variables.

app/assets/stylesheets/framework/buttons.scss

@@ -148,8 +148,8 @@
 
   &.btn-xs {
     padding: 2px $gl-btn-padding;
-    font-size: $gl-btn-small-font-size;
-    line-height: $gl-btn-small-line-height;
+    font-size: $gl-btn-xs-font-size;
+    line-height: $gl-btn-xs-line-height;
   }
 
   &.btn-success,

app/assets/stylesheets/framework/variables.scss

@@ -369,7 +369,9 @@ $gl-btn-line-height: 16px;
 $gl-btn-vert-padding: 8px;
 $gl-btn-horz-padding: 12px;
 $gl-btn-small-font-size: 13px;
-$gl-btn-small-line-height: 13px;
+$gl-btn-small-line-height: 18px;
+$gl-btn-xs-font-size: 13px;
+$gl-btn-xs-line-height: 13px;
 
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 * Badges