RFC: Implement header and footer in dashboard file viewer

[reece]

Goal

Enable notebook authors to provide per-folder information by enabling header and/or footer documents in the file viewer.

Proposal

• The header will be placed just below #tabs div and above #notebook_toolbar div (i.e., above "Select items to perform actions"). The rationale for this placement is that the folder scope applies only to files and not to Running or Clusters tabs.
• The footer will be placed below the #notebook_list div.
• The header and footer are both optional.
• The header will be obtained from the first available file in this order: .header.html, .header.md, header.html, header.md.
• The footer will be obtained from the first available file in this order: .footer.html, .footer.md, footer.html, footer.md.
• If multiple header or footer files are found, only the first in the above order will be used.

Comments/Questions

• The header and footer should be static documents.
• README-based filenames (e.g., README.md) should be reserved for longer explanatory documents.
• index-based filenames (e.g., index.ipynb) should be reserved for main content.
• How important is it that the header and footer be editable within the folder? Other options are to open new markdown editor page or to require editing externally.
• Should the proposed header and footer be moved to the top and bottom rows of the #notebook_list instead (within list_item divs)? Doing so provides better scope of the header and footer under the indicated path, but clutters the file display.

Mockups

Outside notebook_list

Inside notebook_list

Related: Brief exchange with @minrk at https://gitter.im/jupyter/notebook?at=5ba89537e5c2cc56add5adca

Suggested labels: component:File Browser, status:Needs Discussion, type:Enhancement, tag:Design and UX