Rename side panel classes

The names for side panel base classes is confusing. These changes do a few things:

* Split the container out to it's own file side_panel_container.html.
* Renames the container from TimelineViewSidePanelContainer to SidePanelContainer
* Creates a SidePanelRegistry where panels get registered
 * This is, effectively, everything that was previously in TimelineViewSidePanel
* Removes TimelineViewSidePanel since it had no actual functionality
* Changes inheriting classes to inherit HTMLUnknownElement.

TBR=@natduca