Add GroupFinder for collection all groups user has access to

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

app/finders/groups_finder.rb

+class GroupsFinder
+  def execute(current_user, options = {})
+    all_groups(current_user)
+  end
+
+  private
+
+  def all_groups(current_user)
+    if current_user
+      if current_user.authorized_groups.any?
+        # User has access to groups
+        #
+        # Return only:
+        #   groups with public projects
+        #   groups with internal projects
+        #   groups with joined projects
+        #
+        group_ids = Project.public_and_internal_only.pluck(:namespace_id) +
+          current_user.authorized_groups.pluck(:id)
+        Group.where(id: group_ids)
+      else
+        # User has no group membership
+        #
+        # Return only:
+        #   groups with public projects
+        #   groups with internal projects
+        #
+        Group.where(id: Project.public_and_internal_only.pluck(:namespace_id))
+      end
+    else
+      # Not authenticated
+      #
+      # Return only:
+      #   groups with public projects
+      Group.where(id: Project.public_only.pluck(:namespace_id))
+    end
+  end
+end