Fix typo

Thanks @giffmex
Move the wikilink regexps into global config
Fix link dragging to text editors

We were just adding double square brackets if the title contains
spaces. We should be adding them if the title is not a valid wikilink.
Add 2px rounded corners to tiddlers and tabs

An experiment; comments welcome.
Move docs for 5.1.5 release
Release note updates
Adjust 1px error on sidebar tab positioning
Add introduction video
Let people know about the translators edition
Docs tweaks
Docs tweaks
Update readme
Version number update for 5.1.5
Adjust 5.1.5 release date