Remove requirements to use additional properties for Internal Events

Problem

The label, property, value names for Internal events' additional properties can be confusing. It would be beneficial to make it possible for users to use custom names for them, eg. import_type:.

Desired Outcome

It's possible to pass custom additional properties, eg:

Gitlab::InternalEvents.track_event(
  'create_import_access_level',
  category: self.class.name,
  user: current_user,
  additional_properties: { 
    user_role: user_role(current_user, target_namespace),
    import_type: 'bitbucket' 
  }
)

Potential Solution

For the backend, we should be able to just revert !146463 (merged) and !146474 (merged) .

For frontend, we can try to pass the attribute mappings into document js object (proposed here).

How to verify

Further actions needed