Use the merge train to automatically update auto-deploy branches with changes from master

Using the Merge Train project we can automatically merge master into an auto-deploy branch, removing the need to do so manually. For this to work, we need to make the following changes:

1. Set up a Release Tools schedule that takes the latest auto-deploy branch, and triggers a merge-train pipeline
2. Make sure this pipeline performs a security merge, so it fails when there are conflicts
3. Run this job every 15 minutes
4. Make sure this job doesn't schedule any pipelines if there are no changes. This can be done by just checking that both master and the auto-deploy branch have the same last commit, using the API.