Rename LoginBox to AuthButtons

This component displays login and register buttons. It's currently used in the
composer when viewing a room as a guest.

The name is confusing because the login flow uses are very similarly named
`mx_Login_box` as a wrapping around forms, which is totally different than these
buttons.

Additionally, the components is moved to `views/auth` since it is very simple
and auth related.

src/components/structures/RoomView.js

@@ -1757,8 +1757,8 @@ module.exports = React.createClass({
         }
 
         if (MatrixClientPeg.get().isGuest()) {
-            const LoginBox = sdk.getComponent('structures.LoginBox');
-            messageComposer = <LoginBox />;
+            const AuthButtons = sdk.getComponent('views.auth.AuthButtons');
+            messageComposer = <AuthButtons />;
         }
 
         // TODO: Why aren't we storing the term/scope/count in this format