Retrieves the currently logged-in member.
getMember() returns a Promise that resolves
to the currently logged-in member, or undefined if a member isn't logged in.
Notes:
- The member data in the resolved promise will only include custom fields from a site's contacts if those fields are added to a site's members in a dashboard.
- The frontend Members APIs aren't fully functional when previewing a site. View a published version of a site to see their complete functionality.