Feature hugo migration

This MR converts the existing website into a hugo website.

• Converting existing pages into a hugo website
• Existing .html links should work
• Atom feed: Still has validation errors
• html5: Fix all validation errors
• Hide emails from spam bots
• rel="noreferrer" attribute missing on all links
• rel="noopener" attribute missing on all links

Have a look at README.md how to test it with hugo's internal webserver.

This is my very first MR - feedback is very welcome.

If this is not the right way to review and discuss, please let me know.

Closes #1 (closed)

assets/css/styles.css

@@ -10,11 +10,10 @@ body, html {
     line-height:1.3em;
 }
 
-body#index a#indexnav,
-body#download a#downloadnav,
-body#how a#hownav,
-body#about a#aboutnav,
-body#involved a#involvednav {color:#87C214;}
+#nav #is-active {
+  color: #87C214;
+}
+
 
 #wrapper {
     min-height:100%;
@@ -397,7 +396,10 @@ figcaption {
     display:inline;
 }
 
-#flattr {font-size:0.6em;}
+#flattr {
+    font-size:0.6em;
+    text-align:center;
+}
 
 #flattr a {
     margin-right: 1em;
@@ -532,6 +534,7 @@ span.reverse {
 
     #playbutton, #fdroidbutton {
         width:200px;
+        border: 0;
     }
 
     #phone {