Vector is currently in the process of revising the content under the title. This caused breakage in certain gadgets (T315639) and highlighted a need for a more reliable stable API
Currently gadgets add content under the title by targetting the #contentSub element. The #contentSub element however is not always present - it just so happens various skins decided to mark up elements this way.
Going forward similar to mw.util.$content we should have an element that always represents the subtitle no matter what ID it has or it's position in the DOM.
# QA steps
Run the following code:
```
var node = document.createElement('div');
node.textContent = 'hello world!';
mw.util.addSubtitle(node)
```
it should add hello world to the subtitle area (under the title).
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