In addition to changing the button on the "Invite your colleagues"
banner to open the Invite Members modal, instead of being a link to
the Members page, this change:
- removes the :invite_members_group_modal feature flag check from the
  haml partial group page, as it was preventing the modal from opening
- removes the helper method 'can_invite_members_for_group' as it was no
  longer needed
- expands manage_members tests to cover both feature flag scenarios

Changelog: added