Add jinja2 relative links to pages

src/lib/pug/navigation.pug

@@ -1,6 +1,6 @@
 nav.navbar.navbar-expand-lg.navbar-dark.bg-dark
   .container
-    a.navbar-brand(href="index.html") tlater
+    a.navbar-brand(href="{{ url_for('index') }}") tlater
     button.navbar-toggler(
       type="button",
       data-toggle="collapse",
@@ -13,5 +13,5 @@ nav.navbar.navbar-expand-lg.navbar-dark.bg-dark
 
     #navbar.navbar-collapse.collapse
       ul.navbar-nav.mr-auto
-        li.nav-item: a.nav-link(href="mail.html") E-Mail
+        li.nav-item: a.nav-link(href="{{ url_for('mail') }}") E-Mail
         li.nav-item: a.nav-link(href="https://www.github.com/TLATER") GitHub