Add validation message for username regex rule
Add validation message for username regex rule

See flarum/core@445517ee84c15a9f1efd523766b9374d2f327484

flarum/flarum-ext-english,ogioncz/flarum-ext-czech,pladask/flarum-ext-norwegian-bokmal,gtugablue/flarum-ext-portuguese,balkanje/flarum-ext-bosnian1,FlarumChina/flarum-ext-simplified-chinese-pro,ogioncz/flarum-ext-czech,pladask/flarum-ext-norwegian-nynorsk,FlarumChina/flarum-ext-simplified-chinese-pro,raweba/flarum-ext-danish,pladask/flarum-ext-norwegian-nynorsk,flarum/english,pladask/flarum-ext-norwegian-bokmal