Set bottom padding of maximum height of footer

* see #11

Signed-off-by: Jim Martens <github@2martens.de>

_includes/footer.html

@@ -1,7 +1,7 @@
 <footer class="site-footer small text-muted py-3 fixed-bottom">
     <div class="container">
         <div class="row">
-            <div class="col-4">
+            <div class="col-sm-4">
                 <nav class="nav">
                     <ul class="nav">
                         {% for item in site.data.footer-menu.items %}
@@ -13,7 +13,7 @@
                     <img src="/assets/images/uberspace-badge-white.png" />
                 </a>
             </div>
-            <div class="col-8">
+            <div class="col-sm-8">
                 <ul class="list-unstyled">
                     {% if site.data.author.github_username %}
                     <li>