use a custom 404 page

to customize the 404 page you can copy the handlebar template
`src/static/templates/404.hbs` to the TEMPLATES_FOLDER (defaults to
`data/templates/`)

src/config.rs

@@ -1123,6 +1123,8 @@ where
     reg!("admin/organizations");
     reg!("admin/diagnostics");
 
+    reg!("404");
+
     // And then load user templates to overwrite the defaults
     // Use .hbs extension for the files
     // Templates get registered with their relative name