[feature] Customizable media cleaner schedule

[tsmethurst]

Description

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This pull request implements customizable start time + interval period for media cleanup jobs.

Adds media-cleanup-from and media-cleanup-every config fields, which keep the current default values of 00:00:00 (midnight) and 86400000000000 or 24h (24 hours).

These fields allow admins to run their cleanup at different times and rates. For example, say one wanted to run a cleanup every 3 days at 4.30am, one could set media-cleanup-from to 04:30:00 and media-cleanup-every to 72h.

To make this work, instantiation of the cleaner has been pushed all the way back to the server process. This is a bit more complex, code-wise, but has the advantage of the scheduler not being run by default when just doing CLI commands and whatnot.

Still a draft because I want to add some documentation for this.

closes #2212

Checklist

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[daenney]

I would remove the :ss part. I think it's very unlikely anyone needs to control the start time down to the second.

[tsmethurst]

Good point! Changed here: fe1fe95