Impression tracking

Problem/Motivation

View count only tracks views of the canonical. In search, entities may be 'seen' and it would be useful to track.
There are other scenarios in which knowing when, where, and how many times an entity has been presented to users is important.

Steps to reproduce

Proposed resolution

Create an impression table, visitors_impression. It will have columns for entity type, bundle, and entity id. It will join the visitors_log table.

Remaining tasks

User interface changes

API changes

Data model changes