Change preflang field in RegisterForm to a ChoiceField

[dirkcuys]

Change the preflang field in RegisterForm to use a ChoiceField rather than a CharField. Make sure to set the initial value to the current locale (eg. if the user is browsing /en/register/ the initial value should be english)

See https://github.com/p2pu/lernanta/blob/master/lernanta/apps/users/forms.py#L124