Group managers doesn't receive notifications about group membership requests

Problem/Motivation

When a user requests a group membership, the group managers/admins don't receive any notifications about it, even if the email settings are set to immediately.

Steps to reproduce

1. Create a new flexible group with visibility set to the community.
2. As a separate user, go to this group and request a group membership.
3. As group manager, check if you received any notifications about it.

Proposed resolution

The issue is that, when a user creates a group, it receives a Group Admin role, but the code determining who should receive membership request notifications only sends notifications to users with group manager roles.

Remaining tasks

User interface changes

API changes

Data model changes