Accept HTML in update messages and render them in an HTML sheet

[FichteFoll]

By using an HTML sheet, PC could allow plugin authors to display more rich messages after updating with clickable links and inline images, for example.
They could also run commands from links that allow configuring their package.
For compatibility, messages that are not markdown could just be html-escaped and embedded as-is.

Problems

1. Sheets are a new feature, as are command links. For backwards compatibility, a phantom could be used for rendering, but commands would just not work for older versions.
2. Most current change messages are formatted in Markdown and don't use HTML. While we have the python-markdown dependency available, I wouldn't know how to feel about PC depending on the dependencies it makes available in the first place.

[deathaxe]

It would require ST to add an API to query for existing sheets and identify them (e.g. via Sheet.name()).

Otherwise any created sheet, we the instance is lost for can't be closed or re-used anymore. It would become a zombie.

blocked by sublimehq/sublime_text#3826