Register new Arbitrables with the Arbitrator for indexing on the fly with no subgraph change or redeploy

[jaybuidl]

Rationale

We cannot start indexing Arbitrable contracts newly deployed in the wild before they have interacted with a known already indexed contract. We also cannot modify and redeploy the subgraph every time that a new Arbitrable contract has been deployed. So far we have used the DisputeTemplateRegistry as a fixed contract to partially address this indexing challenge: it covers the need to index dispute templates and mappings, but not DisputeRequest events.

Registering all Arbitrables first with the Arbitrator allows a subgraph to start indexing future events from the Arbitrables. Consequently the DisputeTemplateRegistry could be removed (inlined inside arbitrables or as a library).

In a crosschain context, the Arbitrator is the ForeignGateway.

Resolves #1316 #1317