[ENHANCEMENT] datetime of first creation of HashList

[rixvet]

When doing maintenance or statistics (via API/UI) it's useful to know when a hashlist is first created.

This could be for example be done by adding an read-only field date_added TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP on creation which is shown in UI/API.

It might also be a good feature for other objects like File,PreTask and SuperTask.