Add ResourceLinkEvent model and its table

What does this MR do and why?

Related to #395491 (closed)

Issues (aliased as Work Items; WorkItem and Issue are both backed by issues table) can be linked to each other. An example would be adding a Task to an Issue (https://docs.gitlab.com/ee/user/tasks.html#add-existing-tasks-to-an-issue.) The linkings are captured in the model WorkItems::ParentLink.

We want to track the creation and deletion events of WorkItems::ParentLink by modeling the event as ResourceLinkEvent. In this MR, the model and its DB table are added.

MR acceptance checklist

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