fix: show "My Membership" links in My Account menu for team members

[dkoo]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Allows the "My Membership" menu item and its submenu items to be shown to team members only. Team members who join a team by invitation have no other way to view or manage their team membership.

How to test the changes in this Pull Request:

1. On this branch, create a team and assign an owner or manager
2. Confirm that a regular reader (not a member or owner/manager of any team) and the team owner/manager do not see the "Memberships" or "My Membership" menu item in My Account.
3. As the team owner/manager, invite another reader by email or link to join the team.
4. In a new session, join the team via the invitation from step 3.
5. Confirm that as the team member, you do see the "My Membership" menu item and its submenu items in My Account.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?