Add 'Leave Group' link

The link was removed in !3798, probably by mistake.
Signed-off-by: Rémy Coutable <remy@rymai.me>

app/views/layouts/nav/_group_settings.html.haml

 - if current_user
-  - if access = @group.users.find_by(id: current_user.id)
+  - can_edit = can?(current_user, :admin_group, @group)
+  - member = @group.members.non_request.find_by(user_id: current_user.id)
+  - can_leave = member && can?(current_user, :destroy_group_member, member)
+
   .controls
     .dropdown.group-settings-dropdown
       %a.dropdown-new.btn.btn-default#group-settings-button{href: '#', 'data-toggle' => 'dropdown'}
         = icon('cog')
         = icon('caret-down')
       %ul.dropdown-menu.dropdown-menu-align-right
-          - if can?(current_user, :admin_group, @group)
         = nav_link(path: 'groups#projects') do
-              = link_to projects_group_path(@group), title: 'Projects' do
-                Projects
+          = link_to 'Projects', projects_group_path(@group), title: 'Projects'
         %li.divider
+        - if can_edit
           %li
-              = link_to edit_group_path(@group) do
-                Edit Group
+            = link_to 'Edit Group', edit_group_path(@group)
+        - if can_leave
+          %li
+            = link_to polymorphic_path([:leave, @group, :members]),
+              data: { confirm: leave_confirmation_message(@group) }, method: :delete, title: 'Leave group' do
+              Leave Group

spec/features/groups/members/last_owner_cannot_leave_group_spec.rb

+require 'spec_helper'
+
+feature 'Groups > Members > Last owner cannot leave group', feature: true do
+  let(:owner) { create(:user) }
+  let(:group) { create(:group) }
+
+  background do
+    group.add_owner(owner)
+    login_as(owner)
+    visit group_path(group)
+  end
+
+  scenario 'user does not see a "Leave Group" link' do
+    expect(page).not_to have_content 'Leave Group'
+  end
+end

spec/features/groups/members/member_leaves_group_spec.rb

+require 'spec_helper'
+
+feature 'Groups > Members > Member leaves group', feature: true do
+  let(:user) { create(:user) }
+  let(:owner) { create(:user) }
+  let(:group) { create(:group, :public) }
+
+  background do
+    group.add_owner(owner)
+    group.add_developer(user)
+    login_as(user)
+    visit group_path(group)
+  end
+
+  scenario 'user leaves group' do
+    # find('#group-settings-button').click
+    click_link 'Leave Group'
+
+    expect(current_path).to eq(dashboard_groups_path)
+    expect(group.users.exists?(user.id)).to be_falsey
+  end
+end

spec/features/groups/members/user_requests_access_spec.rb

@@ -21,6 +21,7 @@ feature 'Groups > Members > User requests access', feature: true do
     expect(page).to have_content 'Your request for access has been queued for review.'
 
     expect(page).to have_content 'Withdraw Access Request'
+    expect(page).not_to have_content 'Leave Group'
   end
 
   scenario 'user is not listed in the group members page' do