Add instance configuration properties to the schema

[sphars]

With the eventual merging of ferrit's changes into libreddit, there is a now an environment variable for configuring an instance to be SFW-only (see the PR in ferritreader/ferrit#24). It would be a good idea to add a property field to the schema indicating if any instance has these configurations applied. For example, if someone wants to use a SFW-only instance, they know which ones to visit (or avoid). I am planning on standing up a public SFW-only instance once the ferrit changes are merged in.

Thanks!