Allow cronjobs to be executed by calling the built binary

[wilzbach]

• add a main method
• add a CLI flag to call the cron job --run
• configure the heroku addon
  https://devcenter.heroku.com/articles/scheduler

[MartinNowak]

That's a weird cron, but workable. How often do we get migrated on Heroku? You could always run on full hours if you happen to be on at that time.
Also an anacron approach would work but requires some state.

[wilzbach]

Currently the cron runs every six hours:

[CyberShadow]

So as discussed on Slack, this seems to have fallen off at some point.

I got it working again, though, it seems to take a very long time to run. Even with Vibe.d connection persistence fixed, it's been running for three hours and hasn't even finished going through the DMD pull requests. This may need some adjustments going forward.

[CyberShadow]

I got it working again, though, it seems to take a very long time to run.

Figured it out, it was getting stuck in an infinite loop because it wasn't parsing the "Link" header properly:
CyberShadow@01ebb41