api keys: add IP whitelist and API permission system.

Add the ability to restrict API keys so that they can only be used with
certain IP addresses or certain API endpoints.

Restricting your key is useful to limit damage in case it gets leaked or
stolen. For example, if your key is on a remote server and it gets
hacked, or if you accidentally check-in your key to Github.

Restricting your key's API permissions is useful if a third-party app or
script wants your key, but you don't want to give full access to your
account.

If you're an app or userscript developer, and your app needs an API key
from the user, you should only request a key with the minimum
permissions needed by your app.

If you have a privileged account, and you have scripts running under
your account, you are highly encouraged to restrict your key to limit
damage in case your key gets leaked or stolen.

db/structure.sql

@@ -454,7 +454,13 @@ CREATE TABLE public.api_keys (
     user_id integer NOT NULL,
     key character varying NOT NULL,
     created_at timestamp without time zone NOT NULL,
-    updated_at timestamp without time zone NOT NULL
+    updated_at timestamp without time zone NOT NULL,
+    name character varying DEFAULT ''::character varying NOT NULL,
+    permissions character varying[] DEFAULT '{}'::character varying[] NOT NULL,
+    permitted_ip_addresses inet[] DEFAULT '{}'::inet[] NOT NULL,
+    uses integer DEFAULT 0 NOT NULL,
+    last_used_at timestamp without time zone,
+    last_ip_address inet
 );
 
 
@@ -7957,6 +7963,7 @@ INSERT INTO "schema_migrations" (version) VALUES
 ('20210123112752'),
 ('20210127000201'),
 ('20210127012303'),
-('20210214095121');
+('20210214095121'),
+('20210214101614');