Implement parent-child relationship between activities

• Type: Technical Task
• Status: Closed
• Priority: Minor
• Resolution: Completed
• Affects Version/s: 6.2.0 CE M6
• Fix Version/s: 6.2.0 CE M6
• Component/s: Activities widget, Content Publishing > Export/Import, Social Activity, Social Networking
• Labels: None
• Git Pull Request: https://github.com/brianchandotcom/liferay-portal/pull/11352

There are certain situations where we want to throw away or just not show messages on child entities when there is an activity of the same type for their parent. This is true for DELETE events too.

The solution is to introduce two new columns into SocialActivity:

1. parentClassNameId
2. parentClassPK

We are not using the activityId because in most cases we process children first and in those cases we wouldn't have a parent activity ready.