On master: new restricted domain list doesn't respect 'Send Now' button

[rlotun]

Describe the bug
With the new restricted domains list, I can still send dashboard subscription emails using Send Now to an email address whose domain is not on the list.

To Reproduce
Steps to reproduce the behavior:

1. Add a user with an email outside of the domain you want to filter down to - e.g. a persona email address (through Admin)
2. Go to Admin -> General
3. Set a domain in the "Approved Domains for Notifications" box (e.g. metabase.com)
4. Go to a dashboard, click share, and click add an email notification and 'Send Now'.
5. See email sent to account where it shouldn't.

Expected behavior
It shouldn't sent, and should probably show an error saying 'Domain not allowed, contact administrator' or something to that effect.

This happens on the current master.

[jeff303]

Feature added in #17897.

[jeff303]

Added the backend change and test under #18121.

I'm not sure if we'll also want some additional frontend piece to explain what went wrong. With this, the user will just see the button text temporarily change from Send email now to Sending failed (i.e. the current behavior if anything goes wrong with the test send, I suppose).

[camsaul]

Ok, not actually fixed yet. #18122 fixed this for Alerts but not for Dashboard Subscriptions -- this uses a different API endpoint

[nemanjaglumac]

Tested manually for dashboard subscriptions "Send now'.

It shows "Send failed", and it responds with 403. All good.

Do we also want to display the message to the user, like we do when one wants to create a subscription (clicking on the "Done" button)? It feels weird to have it for one case, but not for the other one.

cc @rlotun @brunobergher

[brunobergher]

💯, we should also communicate something to the user in this context. I suspect ideally the code path is the same, right?