Display button groups using Primer flex utilities

This small improvement just uses Primer `d-flex` utilities to `display:
flex;` instead of using style tags. It's preferable.

lib/gollum/templates/navbar.mustache

@@ -18,7 +18,7 @@
     </div>
 
     <div class="TableObject-item hide-sm hide-md">
-      <div class="BtnGroup" style="display: flex;">
+      <div class="BtnGroup d-flex">
         {{#overview}}
           <a
             class="btn BtnGroup-item btn-sm"
@@ -42,7 +42,7 @@
     </div>
 
     <div class="TableObject-item px-2">
-      <div class="BtnGroup" style="display: flex;">
+      <div class="BtnGroup d-flex">
         {{#history}}
           <a
             class="btn BtnGroup-item btn-sm hide-sm hide-md"