Set dependabot to update frontend deps once a month (codeforpdx#565)

Frontend deps are now checked once a month. Security updates will
still be applied immediately. This is because we do not have the
bandwidth to handle frontend dep updates on a weekly basis. The number
of allowed PRs dependabot is allowed to have open is also reduced to 5.

This means that at any given time we should have no more than 5 PRs
(exluding security updates) open at any given time. Plus the one month
review cycle will give us time to thoroughly review the dep update.

.dependabot/config.yml

@@ -10,7 +10,7 @@ update_configs:
 
   - package_manager: "javascript"
     directory: "/src/frontend"
-    update_schedule: "live"
+    update_schedule: "monthly"
     default_labels:
     - "frontend"
     - "dependencies"