Warn users during Sign Up that nicknames can't have spaces

We have run into the problem where non-digital natives try to use spaces in the Sign Up web form and then cannot create an account.

Neither the English-language text in the Sign Up form @nickname field, nor the error message explain tells them that they can't use spaces.

We have temporarily updated the Terms and Conditions as a way of notifying users and will hopefully be able to correct this once we start working with the code directly, but it seems this should be in the default configuration as it's been a surprisingly common problem for us.

But we're loving the platform! Great work.