api/views: Better separate summary from events queries

[victorges]

I found out after the initial version that the viewership summary and viewership
events tables don't actually have the same data: the summary one has the
prometheus data backward compatibility while the events one does not.

To make that distinction clearer to the end-user, added a new metric on the
summary query responses called legacyViewCount, which contains the
number of views that have been migrated from the prometheus data and
thus will not show up in the detailed query endpoints.

Also removed a refactor that did a cheaper summary query when the
events query was made with just a playback ID. This is not possible anymore,
since the tables contain different data, so we cannot replace one query with
the other.

The coded ended up a little simpler tho, without magic attempt to detect what
kind of query it is, which could also lead to unexpected bugs like something
changing and we stopping the use of the summary table altogether.

[gioelecerati]

LGTM