Event summaries

Event summaries: endpoint, migration, store and UI. One row per event type with its totals, instead of asking for the raw events every time you open the page.

I also had to fix the session tops. They were built as refreshable views, which is the same mistake I keep paying for in this project, so now they are insert-triggered like the rest. Sessions needed a dedup on sessions_completed on top, a session can be completed more than once and I was counting it twice.