Add command line interface

[james-cnz]

This pull request removes safety checking on the purge method, on the assumption that this checking will be performed in the user interface, and adds a basic CLI that allows directly calling the method. #5 is needed first, to prevent possible data loss that may result from removing the safety checking.

[danmarsden]

Hey James - I'm not a big fan of the removal of this check - it feels too easy for us to be able to trigger a deletion (even from the front end) of non-assignment files with this change. What's the use-case for allowing users to delete other files in the site with this script?

[james-cnz]

Hi @danmarsden,
Sorry, I've apparently misunderstood something you said.
In Catalyst WR #459151 you wrote "feel free to add a PR with a CLI script that allows an admin - who hopefully knows what they are doing! :-) to run a CLI clean up for any time frame on the fileareas/components/course required."
I took that to mean remove the safety checks, and allow the admin to do anything.
I don't actually have a use case for this.

[danmarsden]

Cool, let's leave that check in for now (only supporting assignment fileareas) - I think removing the check there still opens up a POST in the ui passing a non supported component and could allow someone to delete stuff they should not be deleting in this manner

[james-cnz]

OK, that check's back in.
The minimum age check is still removed, though, and I think it is important that #5 is done first. I think the user interface currently can unintentionally request deletion of files with a minimum age of 0 years, so applying this change first, which removes that check, could lead to real data loss.