Add a date column to voting history page

[leolower]

The date should show the date the latest vote took place for each poll and every vote for each executive.

How to test

• Check that each participation has a date
• Use a query to get vote events and check that the last date is shown in the UI. Also check in etherscan that all vote events are being picked up by the subgraph.

[cmalfesi]

QA test: It's working well.