[UI] Implement markdown editor for schema version descriptions

[wwerner]

While we could have users write their schema version descriptions in markdown in a textarea, we could also integrate an editor w/ buttons for the casual markdowner & a preview.

There are already components out there that would work, but I haven't used them and can't say anything about their quality. E.g. https://github.com/DCsunset/vuetify-markdown-editor

OTOH, if we implement #34, we could probably use this editor for both the schema version spec and the description.

[VaughnVernon]

@wwerner How do we safely and reliably filter markdown from the specifications for compiling to target source types?

[VaughnVernon]

@wwerner Will #34 fix this?

[wwerner]

@VaughnVernon, thanks for the heads up. Yes, this is closed with #48