Customize collapse function by year

so there are no confusing warnings like this

```
ℹ In argument: `voted_pres_08 = clps_pres08(voted_pres_08)`.
Caused by warning:
! 1 unknown level in `f`: Did not Vote
50: There was 1 warning in `mutate()`.
ℹ In argument: `voted_pres_12 = clps_pres12(voted_pres_12)`.
Caused by warning:
! Unknown levels in `f`: Did Not Vote, I Did Not Vote
```