Template out the new task runner interface

database/db.go

database/json_value.go

database/table_tasks.go

migrations/21_not_null_end_ts_background_tasks_down.sql

migrations/21_not_null_end_ts_background_tasks_up.sql

notifier/tasks.go

notifier/uploads.go

tasks/all.go

tasks/exec.go

tasks/schedule.go

tasks/task_runner/purge_previews.go

tasks/task_runner/purge_remote_media.go

tasks/task_runner/purge_thumbnails.go