Let's make a timer that automates Tue "stage-train" tasks
**Acceptance Criteria**
* [ ] Agree on a time for automated deployment every week (selected Tuesday 02:30 UTC, about 20 minutes after the branch cut completes)
* [ ] Patch the deployment calendar to add a new window (along with where to watch for failures)
* [X] User exists for running timer (T303857)
* [ ] Timer exists on deployment servers
* [ ] Timer does not require any human intervention
* [X] Timer determines the wmf.X branch/version for the week (The latest branch found in Gerrit is used)
* [] The right thing happens if the current week's train task is declined.
* [ ] Timer runs `scap stage-train --yes <version>` runs to completion without errors under normal conditions.
* [ ] Set TMUX or STY environment variable to something to avoid an error from check_term_multplxr(), or add an option to skip the check for screen/tmux session.
* [ ] Timer can be re-executed if it previously failed.
* [ ] At least during development, the schedule of the timer must be controllable (at least to ~1 minute resolution) by deployers, including the ability to disable it.
Alerting on failure is T310396