`clear_lastminute_nas` will erase too much data as written for `chng`

Currently, `clear_lastminute_nas` will remove any row which has any `NA`'s in it whatsoever. With multiple signals, this can mean removing rows with real data (as an example, see `11-29-24`, for which `10-02-24` is the last day with data). Before using `chng`, we will need another scheme to handle differently missing data. Ideas:

- locf `NA`'s in non-outcome signals. This is not a great idea, as lags likely become meaningless.
- do the equivalent of `extend_ahead` but for lags (e.g. if the last observation for chng is 30 days behind, adjust the lags of `c(0,7,14)` to `c(30,37,44)`.
- ?

@brookslogan may have other thoughts I forgot about.