Convert rewards_by_day.time field to a timestamp column

[jamiew]

Currently a bigint storing a unixtime

Using timestamp (or time with zone) would allow us to use Metabase's date-filtering logic out-of-the-box

cc @dansku

[jamiew]

BTW I wonder if we would also want to restrict visibility for the rewards table? Nobody really needs <day granularity, and people querying rewards directly will only find a dead-end. I wouln't be surprised if it started causing performance hiccups by pushing better things out of memory etc

[dansku]

I am working on it!
About the rewards restriction, would it be via metabase?