Make management layout similar to admin layout

The <main> tag was including the navigation, and now we use the same
flex layout, making it more accessible for mobile phone users.

I'm not sure the <main> tag should actually include the account info and
the flash message. I'm keeping it like this in order to keep the layout
the way it was.

app/assets/stylesheets/admin.scss

@@ -648,7 +648,7 @@ code {
   border-radius: rem-calc(3);
   font-size: rem-calc(16);
   font-weight: normal;
-  margin: $line-height;
+  margin-bottom: $line-height;
   padding: $line-height / 2;
 
   strong {