Freeze V3 entities/endpoints

I've noticed that even though part of the V3 API entities were moved to a new namespace, some of them are still using V4. Here's a couple of examples:

• https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/api/v3/entities.rb#L6
• https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/api/v3/entities.rb#L19
• https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/api/v3/entities.rb#L70
• https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/api/v3/entities.rb#L95
• https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/api/v3/entities.rb#L114

That makes V3 endpoints change their response whenever the pointed V4 entity is changed. Ultimately, V3 should've been freezed to avoid more reflected changes than expected.

I can think on two options:

1. Freeze all V3 entities data by copying all V4 entities which it's currently pointing to to API::V3::Entities
2. Remove V3 entities/endpoints, which is already expected to happen

cc @to1ne @rymai @smcgivern @DouweM