Use go-humanize package instead of home-grown functions

> Fine for now, but `humanize` package does a good job at making this a list that is easily extended:
> https://github.com/dustin/go-humanize/blob/master/times.go#L47-L65
> 
> It only works for `time.Time` in the package though. Just a nit

_Originally posted by @Emyrk in https://github.com/coder/coder/pull/1828#discussion_r883770893_