remove invalid characters from username field
remove invalid characters from username field

cognitiveclass/edx-platform,cognitiveclass/edx-platform,cognitiveclass/edx-platform,cognitiveclass/edx-platform,bigdatauniversity/edx-platform,bigdatauniversity/edx-platform,bigdatauniversity/edx-platform,cognitiveclass/edx-platform,bigdatauniversity/edx-platform,bigdatauniversity/edx-platform