Run Gregorio online

[rpspringuel]

We might want to add a list of known places to run Gregorio online. We already list two at the beginning of the gabc tutorial, but there are others:

• https://editor.sourceandsummit.com/alpha/
• https://editor.sourceandsummit.com/legacy/ (this one is actually the Illuminare Score Editor's new location, so we should probably update the link in the gabc Tutorial in order to prevent the link from going stale at some point, redirect currently works)
• https://run.gregoriochant.org/cgi/process.pl (alternate location for the Gregorio Chant Engraver that we link to. Does anyone know which is the preferred address?)
• http://bbloomf.github.io/jgabc/transcriber.html