Use Thredded.user_name_column in AtMentionFilter
Use Thredded.user_name_column in AtMentionFilter

Fixes #790

thredded/thredded,jayroh/thredded,jayroh/thredded,jayroh/thredded,thredded/thredded,thredded/thredded,jayroh/thredded,thredded/thredded