add placeholder letters page

app/main/views/letters.py

@@ -0,0 +1,12 @@
+from flask import render_template, abort
+from flask_login import login_required
+
+from app import current_service
+from app.main import main
+from app.utils import user_has_permissions
+
+@main.route("/services/<service_id>/letters")
+@login_required
+@user_has_permissions('manage_templates', admin_override=True)
+def letters(service_id):
+    return render_template('views/letters.html')