get rid of `{janitor}`

[cjyetman]

It's a bit unfortunate that we have this dependency on {janitor} which...

1. we only use for janitor::clean_names(data) and janitor::make_clean_names()
2. we then have to maintain this complex system of undoing whatever those functions do here R/unclean_column_names.R
3. and then we also have to work around a long-standing bug in a dependency of {janitor}, {snakecase}, to avoid cluttering our log with warnings

Unfortunately, getting rid of it is going to be tedious and complicated to adequately consider all the places and ways in which we move back and forth between what {janitor} considers "clean names" and otherwise.

related to #137