Add a stream setting for mandatory email notifications

[alya]

In some cases, administrators may want to require users to receive email notifications for a stream, e.g. when that stream is used for important announcements (as reported in this CZO thread).

We should therefore add an option to require email notifications for a stream.

When a stream has mandatory email notifications:

• Users should be unable to uncheck email notification in the SETTINGS / NOTIFICATIONS UI. There should be a ? next to the checkbox to explain that the stream has mandatory email notifications.
• The notification email should let the user know how they can stop receiving notification emails for the stream, i.e.:
  a) If the user is able to unsubscribe from the stream: "This stream is configured to send email notifications to all subscribers. You can unsubscribe from the stream or deactivate your account to stop receiving these notifications." (with appropriate links)
  b) If the user is unable to unsubscribe from the stream (cf. Add stream setting for who can unsubscribe #19661): "This stream is configured to send email notifications to all subscribers, and your subscription to this stream is required by administrators. You can deactivate your account to stop receiving these notifications."

[zulipbot]

Hello @zulip/server-streams members, this issue was labeled with the "area: stream settings" label, so you may want to check it out!