Prepare for abuse

[bates64]

As @zbanks suggested on Discord, now that we have the scratches appearing on the index page there is potential for abuse.

I'm thinking we have a database singleton that lets us disable a number of features if needed from /admin, without reseting the site:

• Compiling
• Creating new scratches
• Whether /api/scratch returns anonymously-created scratches or not
• Whether /api/scratch returns anything

And also banning/etc of users or IPs

See also #24