separate invite code and email from create account page

[mircealungu]

Note after discussion with @klnyzzz33 @xXPinkmagicXx @gabortodor

One argument for asking for invite code the first thing - then we can already skip the learned language page if the teacher chooses to fix the level, the learned language and even the native language.

[xXPinkmagicXx]

This issue is also related to #955 and #940