[QA'ed but changes made!] Bug/membership request notification

The cuke pretty much explains it all:

```gherkin
Given I am logged in
And I am an admin of a group
And a visitor has requested membership to the group
When I visit the dashboard
And I click the notifications dropdown
Then I should see that the visitor requested access to the group
```

Membership requests from visitors weren't showing up in the notifications drop-down for admins of the group (but they were still adding to the unread notifications count in the nav bar) - resulting in the sticky red badge on notifications that wouldn't go away!

This fixes the issue.