Use custom elements for workspace views

This is the next step in removing dependency on jQuery in core. It replaces all the SpacePen views with custom elements in the workspace. The SpacePen views are left in place as wrapper shims that we will deprecate, but they no longer contain essential logic. For now this uses the `is` attribute with the elements subclassing `div`s for styling compatibility. In another pass we will convert them to custom tag names. The following views have been converted:

* `WorkspaceView` => `WorkspaceElement`
* `PaneContainerView` => `PaneContainerElement`
* `PaneColumnView` and `PaneRowView` => `PaneAxisElement`
* `PaneView` => `PaneElement`
I'll switch it in this PR.