Revert "Merge pull request #843 from code-dot-org/deprecate-username-for-new-users"

This reverts commit 430bef570b67cad216f585e77cceb2ba42866ff4, reversing
changes made to 1c0b2383adaecece09cc37a7cc9c1bd0ab5121b9.
Merge pull request #847 from code-dot-org/revert_843

Revert "Merge pull request #843"