Sanitize RemoteEmail user prop (#27170)

* Sanitize RemoteEmail user prop

If the server is configured to hide user emails, the "RemoteEmail"
user property will be sanitized as well, effectively hiding the real
email of remote users.

* fix merge conflict

---------

Co-authored-by: Doug Lauder <wiggin77@warpmail.net>
Co-authored-by: Mattermost Build <build@mattermost.com>

server/public/model/shared_channel.go

@@ -11,6 +11,11 @@ import (
 	"github.com/pkg/errors"
 )
 
+const (
+	UserPropsKeyRemoteUsername = "RemoteUsername"
+	UserPropsKeyRemoteEmail    = "RemoteEmail"
+)
+
 var (
 	ErrChannelAlreadyShared = errors.New("channel is already shared")
 	ErrChannelHomedOnRemote = errors.New("channel is homed on a remote cluster")