Fix username/mention regexes

* Allow underscore instead of hypen
* Fix domain part handling
* Add tests for remote mention

src/client/app/common/views/components/signup.vue

@@ -76,7 +76,7 @@ export default Vue.extend({
 			}
 
 			const err =
-				!this.username.match(/^[a-zA-Z0-9\-]+$/) ? 'invalid-format' :
+				!this.username.match(/^[a-zA-Z0-9_]+$/) ? 'invalid-format' :
 				this.username.length < 3 ? 'min-range' :
 				this.username.length > 20 ? 'max-range' :
 				null;