user upgrades: add UserUpgrade model.

Add a model to store the status of user upgrades.

* Store the upgrade purchaser and the upgrade receiver (these are
  different for a gifted upgrade, the same for a self upgrade).
* Store the upgrade type: gold, platinum, or gold-to-platinum upgrades.
* Store the upgrade status:
** pending: User is still on the Stripe checkout page, no payment
   received yet.
** processing: User has completed checkout, but the checkout status in
   Stripe is still 'unpaid'.
** complete: We've received notification from Stripe that the payment
   has gone through and the user has been upgraded.
* Store the Stripe checkout ID, to cross-reference the upgrade record on
  Danbooru with the checkout record on Stripe.

This is the upgrade flow:

* When the user clicks the upgrade button on the upgrade page, we call
  POST /user_upgrades and create a pending UserUpgrade.
* We redirect the user to the checkout page on Stripe.
* When the user completes checkout on Stripe, Stripe sends us a webhook
  notification at POST /webhooks/receive.
* When we receive the webhook, we check the payment status, and if it's
  paid we mark the UserUpgrade as complete and upgrade the user.
* After Stripe sees that we have successfully processed the webhook,
  they redirect the user to the /user_upgrades/:id page, where we show
  the user their upgrade receipt.

db/structure.sql

@@ -3104,6 +3104,41 @@ CREATE SEQUENCE public.user_name_change_requests_id_seq
 ALTER SEQUENCE public.user_name_change_requests_id_seq OWNED BY public.user_name_change_requests.id;
 
 
+--
+-- Name: user_upgrades; Type: TABLE; Schema: public; Owner: -
+--
+
+CREATE TABLE public.user_upgrades (
+    id bigint NOT NULL,
+    created_at timestamp(6) without time zone NOT NULL,
+    updated_at timestamp(6) without time zone NOT NULL,
+    recipient_id bigint NOT NULL,
+    purchaser_id bigint NOT NULL,
+    upgrade_type integer NOT NULL,
+    status integer NOT NULL,
+    stripe_id character varying
+);
+
+
+--
+-- Name: user_upgrades_id_seq; Type: SEQUENCE; Schema: public; Owner: -
+--
+
+CREATE SEQUENCE public.user_upgrades_id_seq
+    START WITH 1
+    INCREMENT BY 1
+    NO MINVALUE
+    NO MAXVALUE
+    CACHE 1;
+
+
+--
+-- Name: user_upgrades_id_seq; Type: SEQUENCE OWNED BY; Schema: public; Owner: -
+--
+
+ALTER SEQUENCE public.user_upgrades_id_seq OWNED BY public.user_upgrades.id;
+
+
 --
 -- Name: users_id_seq; Type: SEQUENCE; Schema: public; Owner: -
 --
@@ -4172,6 +4207,13 @@ ALTER TABLE ONLY public.user_feedback ALTER COLUMN id SET DEFAULT nextval('publi
 ALTER TABLE ONLY public.user_name_change_requests ALTER COLUMN id SET DEFAULT nextval('public.user_name_change_requests_id_seq'::regclass);
 
 
+--
+-- Name: user_upgrades id; Type: DEFAULT; Schema: public; Owner: -
+--
+
+ALTER TABLE ONLY public.user_upgrades ALTER COLUMN id SET DEFAULT nextval('public.user_upgrades_id_seq'::regclass);
+
+
 --
 -- Name: users id; Type: DEFAULT; Schema: public; Owner: -
 --
@@ -4537,6 +4579,14 @@ ALTER TABLE ONLY public.user_name_change_requests
     ADD CONSTRAINT user_name_change_requests_pkey PRIMARY KEY (id);
 
 
+--
+-- Name: user_upgrades user_upgrades_pkey; Type: CONSTRAINT; Schema: public; Owner: -
+--
+
+ALTER TABLE ONLY public.user_upgrades
+    ADD CONSTRAINT user_upgrades_pkey PRIMARY KEY (id);
+
+
 --
 -- Name: users users_pkey; Type: CONSTRAINT; Schema: public; Owner: -
 --
@@ -7045,6 +7095,41 @@ CREATE INDEX index_user_name_change_requests_on_original_name ON public.user_nam
 CREATE INDEX index_user_name_change_requests_on_user_id ON public.user_name_change_requests USING btree (user_id);
 
 
+--
+-- Name: index_user_upgrades_on_purchaser_id; Type: INDEX; Schema: public; Owner: -
+--
+
+CREATE INDEX index_user_upgrades_on_purchaser_id ON public.user_upgrades USING btree (purchaser_id);
+
+
+--
+-- Name: index_user_upgrades_on_recipient_id; Type: INDEX; Schema: public; Owner: -
+--
+
+CREATE INDEX index_user_upgrades_on_recipient_id ON public.user_upgrades USING btree (recipient_id);
+
+
+--
+-- Name: index_user_upgrades_on_status; Type: INDEX; Schema: public; Owner: -
+--
+
+CREATE INDEX index_user_upgrades_on_status ON public.user_upgrades USING btree (status);
+
+
+--
+-- Name: index_user_upgrades_on_stripe_id; Type: INDEX; Schema: public; Owner: -
+--
+
+CREATE INDEX index_user_upgrades_on_stripe_id ON public.user_upgrades USING btree (stripe_id);
+
+
+--
+-- Name: index_user_upgrades_on_upgrade_type; Type: INDEX; Schema: public; Owner: -
+--
+
+CREATE INDEX index_user_upgrades_on_upgrade_type ON public.user_upgrades USING btree (upgrade_type);
+
+
 --
 -- Name: index_users_on_created_at; Type: INDEX; Schema: public; Owner: -
 --
@@ -7437,6 +7522,7 @@ INSERT INTO "schema_migrations" (version) VALUES
 ('20200816175151'),
 ('20201201211748'),
 ('20201213052805'),
-('20201219201007');
+('20201219201007'),
+('20201224101208');