Let Gitlab::CurrentSettings read from ApplicationSetting and OrganizationSetting

Currently, Gitlab has instance-wide settings in ApplicationSetting. Some of those settings are relevant for the instance, some for the Organization. So we expect that we will move settings to OrganizationSetting model.

In the code base, Gitlab::CurrentSetting is now a entry point for reading instance wide settings. We want to use that class also for OrganizationSetting.

Implementation plan

• Gitlab::CurrentSetting will read setting from ApplicationSetting
• If that setting does not exist, it will read from setting OrganizationSetting
  • !135442 (merged) use Current Organization
  • If we don't know the Current Organization, we return nothing (no fallback to default organization)
• For migration purposes, a setting can exist in both settings. the ApplicationSetting takes precedence

So the priority will not change yet. This is for performance reasons: we have more settings in ApplicationSetting for now. In the future, we can change this and read from OrganizationSetting first.