add Log in/register message for anon users

to join groups.
addresses #15

src/js/GroupInfo.js

@@ -164,7 +164,7 @@ class GroupMembershipActions extends React.Component{
 	}
 	render(){
 		if(!currentUser){
-			return null;
+			return <p><a href={buildUrl('login')}>Log in</a> or <a href={buildUrl('register')}>Register</a> to join groups</p>;
 		}
 		let group = this.props.group;
 		let member = allGroupMembers(group).includes(currentUser.userID);

src/js/wwwroutes.js

@@ -80,6 +80,10 @@ let buildUrl = function(name, params){
 			return '/download/connectors';
 		case 'download':
 			return '/download';
+		case 'login':
+			return '/user/login';
+		case 'register':
+			return '/user/register';
 	}
 	throw new Error('Unknown route in buildUrl');
 };