Add min-height to main app

Adds min-height: 80vh; The footer will stay always at the bottom on big screens and looks normal on mobile devices.

app/static/css/main.css

@@ -7,6 +7,10 @@ select {
   font-family: Arial, Helvetica, sans-serif !important;
 }
 
+#app {
+  min-height: 80vh;
+}
+
 h3.header {
   margin-bottom: 2.5rem;
 }