remote_billing: Implement confirmation flow for RemoteRealm auth.

The way the flow goes now is this:
1. The user initiaties login via "Billing" in the gear menu.
2. That takes them to `/self-hosted-billing/` (possibly with a
   `next_page` param if we use that for some gear menu options).
3. The server queries the bouncer to give the user a link with a signed
   access token.
4. The user is redirected to that link (on `selfhosting.zulipchat.com`).
Now we have two cases, either the user is logging in for the first time
and already did in the past.
If this is the first time, we have:
5. The user is asked to fill in their email in a form that's shown,
   pre-filled with the value provided inside the signed access token.
   They POST this to the next endpoint.
6. The next endpoint sends a confirmation email to that address and asks
   the user to go check their email.
7. The user clicks the link in their email is taken to the
   from_confirmation endpoint.
8. Their initial RemoteBillingUser is created, a new signed link like in
   (3) is generated and they're transparently taken back to (4),
   where now that they have a RemoteBillingUser, they're handled
   just like a user who already logged in before:
If the user already logged in before, they go straight here:
9. "Confirm login" page - they're shown their information (email and
   full_name), can update
   their full name in the form if they want. They also accept ToS here
   if necessary. They POST this form back to
   the endpoint and finally have a logged in session.
10. They're redirected to billing (or `next_page`) now that they have
    access.

zilencer/models.py

@@ -154,17 +154,40 @@ class RemoteRealm(models.Model):
         return f"{self.host} {str(self.uuid)[0:12]}"
 
 
-class RemoteRealmBillingUser(models.Model):
+class AbstractRemoteRealmBillingUser(models.Model):
     remote_realm = models.ForeignKey(RemoteRealm, on_delete=models.CASCADE)
 
     # The .uuid of the UserProfile on the remote server
     user_uuid = models.UUIDField()
-    full_name = models.TextField(default="")
     email = models.EmailField()
 
+    class Meta:
+        abstract = True
+
+
+class RemoteRealmBillingUser(AbstractRemoteRealmBillingUser):
+    full_name = models.TextField(default="")
+
     TOS_VERSION_BEFORE_FIRST_LOGIN = UserProfile.TOS_VERSION_BEFORE_FIRST_LOGIN
     tos_version = models.TextField(default=TOS_VERSION_BEFORE_FIRST_LOGIN)
 
+    class Meta:
+        unique_together = [
+            ("remote_realm", "user_uuid"),
+        ]
+
+
+class PreregistrationRemoteRealmBillingUser(AbstractRemoteRealmBillingUser):
+    # status: whether an object has been confirmed.
+    #   if confirmed, set to confirmation.settings.STATUS_USED
+    status = models.IntegerField(default=0)
+
+    # These are for carrying certain information that's originally
+    # in an IdentityDict across the confirmation link flow. These
+    # values will be restored in the final, fully authenticated IdentityDict.
+    next_page = models.TextField(null=True)
+    uri_scheme = models.TextField()
+
 
 class AbstractRemoteServerBillingUser(models.Model):
     remote_server = models.ForeignKey(RemoteZulipServer, on_delete=models.CASCADE)