fix(ical): do not expose user email addresses in calendar feeds

The project and user iCal feeds only need a token and they put the
email address of the task assignee and creator in ATTENDEE and
ORGANIZER. Always use the username@kanboard.local address that was
already the fallback for users without an email.

This also fixes ORGANIZER for creators without an email, which was
rendered as a bare MAILTO: because of operator precedence.

app/Model/TaskFinderModel.php

@@ -320,10 +320,8 @@ class TaskFinderModel extends Base
             ->left(UserModel::TABLE, 'uc', 'id', TaskModel::TABLE, 'creator_id')
             ->columns(
                 TaskModel::TABLE.'.*',
-                'ua.email AS assignee_email',
                 'ua.name AS assignee_name',
                 'ua.username AS assignee_username',
-                'uc.email AS creator_email',
                 'uc.name AS creator_name',
                 'uc.username AS creator_username'
             );