Add buttons to export / download / view source for the whole notebook

[josevalim]

Since we have a markdown source, we should make it easier to download the markdown source if we want. The big question is where we are going to have those buttons. I believe we have two options right now:

1. Put it under notebook settings in the sidebar

2. Put those as buttons on the notebook title, similar to what we do to sections, see the red circle here:

Another notebook-wide button that we may want to make accessible is the fork button.

[josevalim]

I can also see this being a whole separate tab under settings, called export, especially if we want to support different kinds of export: export as HTML, export as Live Markdown, export to a new notebook (fork), etc. On the other hand, export as HTML it most likely just print and we will leverage print css.

[josevalim]

Another export we may want to have is the export of only the Elixir code. Optionally with the markdown as comments. And optionally with outputs as comments.

[gamache]

I'd love to use Livebook as the basis for a static blog generator, where users have the option to download the .livemd and play with it on their machines. If it were possible to export a fully evaluated notebook, this would be fairly simple.

[Benjamin-Philip]

Speaking of exporting to HTML, I think we should support Pandoc. This will enable users to export to PDF, ipynb, docx etc. Maybe if we have the time, we could even setup Haskell - Elixir bindings to support conversion from our UI!

[josevalim]

Reopening because we should at least add the export options to skip comments and to add outputs, as well as other export types if possible (print/html).