Remove assumptions around auth_events and prev_events

[kegsay]

We want the visualiser to be as agnostic as possible to the underlying event data, so if we experiment with radically different shapes (e.g auth dags) then tardis will Just Work with them.

Current assumptions:

• harcoded checkboxes for auth chain
• hardcoded event ID calculation for prev|auth_events fields and that's it.
• eventsToCompleteDag explicitly loops auth|prev_events to fill in a complete dag.
• findForwardExtremities explciitly walks auth|prev_events when doing its deathmatch calculation.
• collapsifier only looks at prev_events.