Support for Unlisted and Private Snippets

[seveibar]

• Add property is_public to snippets
• Add property is_unlisted to snippets
• Add property is_private to snippets
• Add computed property display_visibility="private" | "unlisted" | "internal" | "public"

(internal is for when we have orgs)

Support displaying these labels on the snippets

Alter what snippets are returned and add tests for e.g. /snippets/list and /snippets/get as well as other endpoints

Can introduce new auth middleware to perform check too!