Schedule Renovate updates once weekly

We already have GitHub's vulnerability scanning enabled for this
repository, which will alert us about vulnerabilities in NPM packages we
use. Everything else can be updated with less urgency.

https://help.github.com/articles/about-security-alerts-for-vulnerable-dependencies/

renovate.json

@@ -1,5 +1,6 @@
 {
     "extends": [
         "config:base"
-    ]
+    ],
+    "schedule": ["before 5am on monday"]
 }