add Nh3Char(models.CharField)

[LouisKerdres]

hello

i need to have a CharField sanitized so I suggest this changes.

I'm not used to make PR on others repo, feel free to ask more modification to fit with your quality policy

rename Nh3Field in Nh3Text and deprecate the old naming too

[marksweb]

@LouisKerdres Apologies for the time it's taking for me to get to this!

I did start looking into the "textbook approach" to deprecating classes, having never actually needed to do that before.

I have also now remembered to assign this to myself so that will (hopefully) help me keep track of it.