Link selection templates get printed twice if content-block is sub-classed

[NateWr]

The content-block component enqueues script assets directly into the footer. If another class extends that class and needs the link panel, it will print the templates twice, and can't avoid doing so if the component should be de-coupled (ie - allow subclassed-content-block but not content-block).

I suspect the link panel will be a useful generic feature, so perhaps it should be split off from content-block and only loaded once no matter how many components call it.

I'm thinking something like a CLC_Content_Layout_Control::features registry which can support global features like this.