Make Thumbs an XBlockAside

[PLAT-215]
The javascript signatures are a bit different for asides than blocks but it may not matter: the constructor takes `runtime, element, block_element, init_args`.

You also need to pass the block's usage id to the aside's frag via `usage_id=block.scope_ids.usage_id,` which `self.student_view` won't be doing.