remote_billing: Fix /self-hosted-billing/ handling for desktop app.

When you click "Plan management", the desktop app opens
/self-hosted-billing/ in your browser immediately. So that works badly
if you're already logged into another account in the browser, since that
session will be used and it may be for a different user account than in
the desktop app, causing unintended behavior.

The solution is to replace the on click behavior for "Plan management"
in the desktop app case, to instead make a request to a new endpoint
/json/self-hosted-billing, which provides the billing access url in a
json response. The desktop app takes that URL and window.open()s it (in
the browser). And so a remote billing session for the intended user will
be obtained.

zerver/views/push_notifications.py

@@ -12,6 +12,7 @@ from zerver.lib.exceptions import (
     MissingRemoteRealmError,
     OrganizationOwnerRequiredError,
     RemoteRealmServerMismatchError,
+    ResourceNotFoundError,
 )
 from zerver.lib.push_notifications import (
     InvalidPushDeviceTokenError,
@@ -118,32 +119,25 @@ def send_test_push_notification_api(
     return json_success(request)
 
 
-@zulip_login_required
-@typed_endpoint
-def self_hosting_auth_redirect(
-    request: HttpRequest,
-    *,
-    next_page: Optional[str] = None,
-) -> HttpResponse:
-    if not uses_notification_bouncer():  # nocoverage
-        return render(request, "404.html", status=404)
+def self_hosting_auth_view_common(
+    request: HttpRequest, user_profile: UserProfile, next_page: Optional[str] = None
+) -> str:
+    if not uses_notification_bouncer():
+        raise ResourceNotFoundError(_("Server doesn't use the push notification service"))
 
-    user = request.user
-    assert user.is_authenticated
-    assert isinstance(user, UserProfile)
-    if not user.has_billing_access:  # nocoverage
+    if not user_profile.has_billing_access:  # nocoverage
         # We may want to replace this with an html error page at some point,
         # but this endpoint shouldn't be accessible via the UI to an unauthorized
-        # user - and they need to directly enter the URL in their browser. So a json
+        # user_profile - and they need to directly enter the URL in their browser. So a json
         # error may be sufficient.
         raise OrganizationOwnerRequiredError
 
-    realm_info = get_realms_info_for_push_bouncer(user.realm_id)[0]
+    realm_info = get_realms_info_for_push_bouncer(user_profile.realm_id)[0]
 
     user_info = UserDataForRemoteBilling(
-        uuid=user.uuid,
-        email=user.delivery_email,
-        full_name=user.full_name,
+        uuid=user_profile.uuid,
+        email=user_profile.delivery_email,
+        full_name=user_profile.full_name,
     )
 
     post_data = {
@@ -164,12 +158,55 @@ def self_hosting_auth_redirect(
         # Upload realm info and re-try. It should work now.
         send_server_data_to_push_bouncer(consider_usage_statistics=False)
         result = send_to_push_bouncer("POST", "server/billing", post_data)
-    except RemoteRealmServerMismatchError:  # nocoverage
-        return render(request, "zilencer/remote_realm_server_mismatch_error.html", status=403)
 
     if result["result"] != "success":  # nocoverage
         raise JsonableError(_("Error returned by the bouncer: {result}").format(result=result))
 
     redirect_url = result["billing_access_url"]
     assert isinstance(redirect_url, str)
+    return redirect_url
+
+
+@zulip_login_required
+@typed_endpoint
+def self_hosting_auth_redirect_endpoint(
+    request: HttpRequest,
+    *,
+    next_page: Optional[str] = None,
+) -> HttpResponse:
+    """
+    This endpoint is used by the web app running in the browser. We serve HTML
+    error pages, and in case of success a simple redirect to the remote billing
+    access link received from the bouncer.
+    """
+
+    user = request.user
+    assert user.is_authenticated
+    assert isinstance(user, UserProfile)
+
+    try:
+        redirect_url = self_hosting_auth_view_common(request, user, next_page)
+    except ResourceNotFoundError:
+        return render(request, "404.html", status=404)
+    except RemoteRealmServerMismatchError:
+        return render(request, "zilencer/remote_realm_server_mismatch_error.html", status=403)
+
     return HttpResponseRedirect(redirect_url)
+
+
+@typed_endpoint
+def self_hosting_auth_json_endpoint(
+    request: HttpRequest,
+    user_profile: UserProfile,
+    *,
+    next_page: Optional[str] = None,
+) -> HttpResponse:
+    """
+    This endpoint is used by the desktop application. It makes an API request here,
+    expecting a JSON response with either the billing access link, or appropriate
+    error information.
+    """
+
+    redirect_url = self_hosting_auth_view_common(request, user_profile, next_page)
+
+    return json_success(request, data={"billing_access_url": redirect_url})