Support setting metadata like title, author and date

Problem

Currently there is no way on the UI to set metadata, such as title, author and date, of a notebook. In consequence, one has to enter such information in the main document, or dedicate one slide to it, using heading 1 for title, sub-headings or paragraphs for author and date. Trouble is, afterwards when s/he wants to convert the notebook to another format, especially PDF, such metadata become part of the main content. In short, the input (notebook) and output (PDF) format are not well aligned.

Suggestion

Support setting metadata like title, author and date in the UI and use them to produce the counterpart in the output format when nbconvert-ing the notebook, so that a document will have a head with title/author/date and slides will have a cover with similarly information.

Start by tracing how notebook metadata is represented in the UI and passed to nbconvert. Check the document and slide output paths, including PDF conversion. Done means title, author, and date can be entered in the UI and appear as document heading metadata or slide-cover information.