Subgraph: Use on-chain registration timestamps instead of IPFS data

## Overview
Currently we store registration times (start/end timestamps) in on-chain events and in IPFS election metadata. This could lead to inconsistency between IPFS and on-chain data.
We should remove the registration times from the IPFS metadata, and return in the subgraph event-based indexed data.


### To-Do:
tbd
