Bad practice: validating username in the save()

Related: https://git.startinblox.com/applications/sib-app/issues/424#note_23457

Related lines: https://git.startinblox.com/djangoldp-packages/djangoldp-account/blob/3b4a90fa81378fafc9ad1da2d7605d9b52e4ea59/djangoldp_account/models.py#L95-100

As a quick fix, we merged these. But we'll need, at some point, to understand why we have this behavior and it may need a rework here.