Handling of usernames with spaces

[hthuman]

We had several students who were unable to login due to "No user with this username exists" errors even though we could find the username in the Snap! site.

Through trial-and-error, we determined that the cause was usernames with spaces. It looks like Snap automatically removes spaces from usernames while still preserving them for display names or searchable names.

Some suggestions that might be less confusing for students:

1. Don't allow usernames with spaces
2. Warn users that their username will be changed when they create it
3. Remove the space for the display name too