cron.py: fix missing import, preserve /h/changelog.

Multiple closely related issues:
  - When sub_inactive_purge_task was moved to cron.py, a number of
    imports were not added. Notably, missing helpers/alerts.py.
    This led to hole inactivity deletions not taking place.
  - However, this proved useful: the hole it's been trying to delete
    the past few days was /h/changelog. This has now been excluded
    from deletion.
  - Finally, the missing import would've meant the monthly procoins
    disbursement would fail when it runs in a couple days. This has
    likewise been fixed.

files/helpers/cron.py

@@ -1,6 +1,7 @@
 from files.cli import g, app, db_session
 import click
 from files.helpers.const import *
+from files.helpers.alerts import send_repeatable_notification
 from files.classes import *
 
 import files.helpers.lottery as lottery
@@ -48,6 +49,7 @@ def sub_inactive_purge_task():
 	one_week_ago = time.time() - 604800
 	active_holes = [x[0] for x in g.db.query(Submission.sub).distinct() \
 		.filter(Submission.sub != None, Submission.created_utc > one_week_ago).all()]
+	active_holes.append('changelog') # system hole immune from deletion
 
 	dead_holes = g.db.query(Sub).filter(Sub.name.notin_(active_holes)).all()
 	names = [x.name for x in dead_holes]