Implement reassign command

app/services/slash_commands/interpret_service.rb

@@ -141,6 +141,21 @@ module SlashCommands
       end
     end
 
+    desc 'Change assignee(s)'
+    explanation do
+      'Change assignee(s)'
+    end
+    params '@user1 @user2'
+    condition do
+      issuable.is_a?(Issue) &&
+        issuable.persisted? &&
+        issuable.assignees.any? &&
+        current_user.can?(:"admin_#{issuable.to_ability_name}", project)
+    end
+    command :reassign do |unassign_param|
+      @updates[:assignee_ids] = extract_users(unassign_param).map(&:id)
+    end
+
     desc 'Set milestone'
     explanation do |milestone|
       "Sets the milestone to #{milestone.to_reference}." if milestone

spec/services/slash_commands/interpret_service_spec.rb

@@ -455,6 +455,22 @@ describe SlashCommands::InterpretService, services: true do
         end
       end
 
+      context 'reassign command' do
+        let(:content) { '/reassign' }
+
+        context 'Issue' do
+          it 'reassigns user if content contains /reassign @user' do
+            user = create(:user)
+
+            issue.update(assignee_ids: [developer.id, developer2.id])
+
+            _, updates = service.execute("/reassign @#{user.username}", issue)
+
+            expect(updates).to eq(assignee_ids: [user.id])
+          end
+        end
+      end
+
       context 'Merge Request' do
         it 'populates assignee_id: nil if content contains /unassign' do
           merge_request.update(assignee_id: developer.id)