Add the ability to ban posts

[Rebel4788]

I don't know if this is a feature this project wants but I intend to use it and you may like the feature. Feel free to dismiss.

This adds the ability to ban uploaded post files and prevent them from being uploaded again.

[noirscape]

I like this in concept, seems very useful to comply with the DMCA and similar copyright laws for public boorus. That said, it seems to be missing a few endpoints:

• Listing banned sha1sums does not seem to be possible at the moment.
• There's similarly, no unban mechanism to lift a ban later down the line.

Would suggest adding an endpoint + frontend stuff for those two things.

[Rebel4788]

Good points. I've added basic endpoints + frontend pages for listing and removing banned posts. Hopefully this is satisfactory.

I'm no expert in this technology stack so this may need a good look to make sure everything is implemented correctly.