fix: interval schedule task start_time

Django celery beat interval scheduler does not start tasks exactly at the given time set in start_time field.

We can make this work by setting last_run_at to start_time - interval.

This MR also removes edit/delete option for interval schedule from recurrence admin page and improves Recurrence, Run and Checklist list display in admin

Supporting information

• Celery beat issue
• Jira: BB-6433

Test instructions

• Create a recurrence with start_date set in future, for example:

• Check the corresponding runs are created at the specified start time.