Remind about escalation policies after creating a rotation

After a schedule is created, a tip alert reminds the user to also create an escalation policy so notifications are sent. Adding the first rotation replaces that tip with a plain "Successfully created a new rotation" alert, so the reminder disappears before the user has acted on it and the schedule can silently end up without notifications.

This MR repeats the escalation policy sentence, with the link to the escalation policies page, inside the success alert when a rotation is created. Editing a rotation keeps the plain message. The rotation modal now emits rotation-updated with a { created: true } option, which oncall_schedule.vue passes through unchanged; the wrapper decides whether to render the reminder.

One new externalised string is added to locale/gitlab.pot.

Resolves #338914

Differences

Before: After adding a rotation the alert reads "Successfully created a new rotation" and the escalation policy tip is gone.

After: The alert reads "Successfully created a new rotation. To enable notifications for this schedule, you must also create an escalation policy." with "escalation policy" linking to the project's escalation policies page. Editing a rotation still shows only "Successfully edited your rotation".

How to set up and Validate locally:

1. Premium/Ultimate license, Monitor > On-call Schedules, create a schedule.
2. Add a rotation: the green alert includes the escalation policy reminder and link.
3. Edit the rotation: the alert has no reminder.
4. yarn jest ee/spec/frontend/oncall_schedule