Add a navbar to the main page after login

I'll probably use it. Fairly certain. It looks nice either way.

This breaks up the main layout to 'things with the navbar' and 'things
without' and uses inheritance to make it all work out nicely. I also
added my first static asset which assumes nginx is properly configured
to serve up the file rather than doing it through vanth

static/css/vanth.css

@@ -0,0 +1,4 @@
+.main-content {
+  position: relative;
+  top: 50px;
+}