Rebuild how failed logs are removed to be a cron

[mgwidmann]

PRs are crashing due to a foreign key constraint error (which is good because that means it cannot delete something it should not).

Task #PID<0.2979.0> started from :"pr-somerepo-12345" terminating
** (Ecto.ConstraintError) constraint error when attempting to insert struct:

    * foreign_key: random_failures_failure_log_id_fkey

If you would like to convert this constraint into an error, please
call foreign_key_constraint/3 in your changeset and define the proper
constraint name. The changeset has not defined any constraint.

    (ecto) lib/ecto/repo/schema.ex:570: anonymous fn/4 in Ecto.Repo.Schema.constraints_to_errors/3
    (elixir) lib/enum.ex:1255: Enum."-map/2-lists^map/1-0-"/2
    (ecto) lib/ecto/repo/schema.ex:555: Ecto.Repo.Schema.constraints_to_errors/3
    (ecto) lib/ecto/repo/schema.ex:219: anonymous fn/14 in Ecto.Repo.Schema.do_insert/4
    (ecto) lib/ecto/repo/schema.ex:768: anonymous fn/3 in Ecto.Repo.Schema.wrap_in_transaction/6
    (ecto) lib/ecto/adapters/sql.ex:576: anonymous fn/3 in Ecto.Adapters.SQL.do_transaction/3
    (db_connection) lib/db_connection.ex:1275: DBConnection.transaction_run/4
    (db_connection) lib/db_connection.ex:1199: DBConnection.run_begin/3
Function: &SlackCoder.Services.RandomFailureService.save_random_failure/1

Rebuild it to periodically clean up randomly failed jobs that are not necessary to be kept around any longer instead of cleaning up the jobs as they are completed.