Show error message when team name is fewer than 4 characters

I have still kept the length validation in team_signup_url_page.jsx in case
user chooses the url shorter than 4 characters

web/react/components/team_signup_display_name_page.jsx

@@ -25,6 +25,9 @@ export default class TeamSignupDisplayNamePage extends React.Component {
         if (!displayName) {
             this.setState({nameError: 'This field is required'});
             return;
+        } else if (displayName.length <= 2 || displayName.length > 15) {
+            this.setState({nameError: 'Name must be 3 or more characters up to a maximum of 15'});
+            return;
         }
 
         this.props.state.wizard = 'team_url';