graph: add timestamp to QFVote

[thelostone-mc]

Defer to zen to add in more context here on usecase

Currently our while graph indexing does not record timestamp of the event fired.
This is an improvement which would help FDD (check description from zen below)
This is a feature request to enable indexing timestamp of vote

@nfrgosselin

[zengatsu]

We are relying heavily on the graph to easily pull data for analysis
and having the timestamps attached to the votes would help with certain types of analysis

This is also useful to make more visualizations on the dashboard

I had added an issue for this here #862

Projects applications also would benefit from the timestamps