Add canned filter for recent system tiddlers

Useful when you need to establish the titles of tiddlers being used for
state storage.
Make tag pills rounder

The tag pills were looking less like pills at larger font sizes,
because of the absolute units used for the corner roundness.
Remove extraneous console.log
Release note update
First pass at new introduction edition

It started life as the slides for a new introductory video, but I’m
thinking that it may be worth publishing as an interactive introduction.