User should be able to leave group. If not - show him proper message

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

CHANGELOG

@@ -29,6 +29,7 @@ v 7.12.0 (unreleased)
   - Clarify navigation labels for Project Settings and Group Settings.
   - Move user avatar and logout button to sidebar
   - You can not remove user if he/she is an only owner of group
+  - User should be able to leave group. If not - show him proper message
 v 7.11.4
   - Fix missing bullets when creating lists

app/controllers/groups/group_members_controller.rb

@@ -66,7 +66,11 @@ class Groups::GroupMembersController < Groups::ApplicationController
       @group_member.destroy
       redirect_to(dashboard_groups_path, notice: "You left #{group.name} group.")
     else
-      return render_403
+      if @group.last_owner?(current_user)
+        redirect_to(dashboard_groups_path, alert: "You can not leave #{group.name} group because you're the last owner. Transfer or delete the group.")
+      else
+        return render_403
+      end
     end
   end

app/views/dashboard/groups/index.html.haml

@@ -23,10 +23,9 @@
               %i.fa.fa-cogs
               Settings
-          - if can?(current_user, :destroy_group_member, group_member)
-            = link_to leave_group_group_members_path(group), data: { confirm: leave_group_message(group.name) }, method: :delete, class: "btn-sm btn btn-grouped", title: 'Remove user from group' do
-              %i.fa.fa-sign-out
-              Leave
+          = link_to leave_group_group_members_path(group), data: { confirm: leave_group_message(group.name) }, method: :delete, class: "btn-sm btn btn-grouped", title: 'Leave this group' do
+            %i.fa.fa-sign-out
+            Leave
         = image_tag group_icon(group), class: "avatar s40 avatar-tile"
         = link_to group, class: 'group-name' do