Browser-based presentation tool with a reST syntax and all m.css goodies

[mosra]

The branch has been around since 2018, just not tested, documented or actually published. Time to change that!

Features:

• Write your slides with the usual reST syntax
  • Meaning you can easily reuse content from presentations to blog posts and vice versa
• Present in a web browser
  • With all the advantages it has, like embedded videos or WebGL content
• Print-only CSS to save as a PDF using a PDF printer in the browser
• Tiny amount of JS to navigate back and forth
• Synchronized presenter view with notes in another window

Things left to do:

• Write tests
• Refresh the docs to what makes sense in 2022, especially regarding PDF print
• Update copyright years, heh
• Cleanup the m.htmlsanity additions
• Figure out a way to enforce use of the .m-*-t CSS classes for advanced layouts, as the .m-*-s, m and l are window-size-dependent and break especially during print (or maybe change it so it behaves the same for all sizes? turn the screen sizes into a variable and change it here to something else?)
• Sync plugin usage / handling with the Python doc generator