page: Simplify template to save modes on page__body. Closes #516

just applied @narqo's patch from https://github.com/bem/bem-core/pull/517#issuecomment-43195972
Maybe we could optimize it a little bit?

```js
var mix = applyNext(),
  uaMix = [{ block : 'ua',.. }];
return mix? uaMix.concat(mix) : uaMix;
```