Remove urth, support v1 metadata spec

[parente]

• Remove all urth references in CSS classes and whatnot
• Adopt v1 metadata spec when https://github.com/jupyter-incubator/dashboards/wiki/Dashboard-Metadata-and-Rendering is finalized
• Decide if/how to support "upgrading" from v0 to v1 metadata in notebooks

DO NOT add any new user-facing features (e.g., NO multi-layout support). This is a cleanup and switcheroo only.

[parente]

https://github.com/jupyter-incubator/dashboards/wiki/Dashboard-Metadata-and-Rendering#spec-v1-draft is done. We can / should start seeing what it takes to switch real soon now.

[parente]

Re: the metadata, the jury is still out on whether the folks want metadata.extensions.your_app or just metadata.extensions.

[dalogsdon]

Doing the upgrading should be a simple matter of copying over existing data when initializing the metadata structure then deleting the old data.

[dalogsdon]

One benefit of this is that we no longer clobber layout data when switching between grid and report layouts.

[parente]

I think we can close this one. We'll open others as needed.