GitHub: Enable dependabot (#2289)

.github/dependabot.yml

@@ -0,0 +1,15 @@
+# To get started with Dependabot version updates, you'll need to specify which
+# package ecosystems to update and where the package manifests are located.
+# Please see the documentation for all configuration options:
+# https://help.github.com/github/administering-a-repository/configuration-options-for-dependency-updates
+
+version: 2
+updates:
+  - package-ecosystem: "pip"
+    directory: "/src"
+    schedule:
+      interval: "daily"
+  - package-ecosystem: "npm"
+    directory: "/src/pretix/static/npm_dir"
+    schedule:
+      interval: "monthly"