streams: Add API endpoint to get stream email.

This commit adds new API endpoint to get stream email which is
used by the web-app as well to get the email when a user tries
to open the stream email modal.

The stream email is returned only to the users who have access
to it. Specifically for private streams only subscribed users
have access to its email. And for public streams, all non-guest
users and only subscribed guests have access to its email.
All users can access email of web-public streams.

zerver/views/streams.py

@@ -47,6 +47,7 @@ from zerver.decorator import (
     require_post,
     require_realm_admin,
 )
+from zerver.lib.email_mirror_helpers import encode_email_address
 from zerver.lib.exceptions import (
     ErrorCode,
     JsonableError,
@@ -1073,3 +1074,18 @@ def update_subscription_properties_backend(
         )
 
     return json_success(request)
+
+
+@has_request_variables
+def get_stream_email_address(
+    request: HttpRequest,
+    user_profile: UserProfile,
+    stream_id: int = REQ("stream", converter=to_non_negative_int, path_only=True),
+) -> HttpResponse:
+    (stream, sub) = access_stream_by_id(
+        user_profile,
+        stream_id,
+    )
+    stream_email = encode_email_address(stream, show_sender=True)
+
+    return json_success(request, data={"email": stream_email})