Markdown tabs

[edemaine]

I often see instructions tabs in Comingle, either embedded as Google Docs or Cocreate boards. It probably makes sense for Comingle to directly support tabs with custom Markdown content, instead of an external URL. The main challenge here is that we finally need an "edit tab" feature, where you could imagine changing a page's URL in addition to its Markdown content (as we can't be expected to get perfect Markdown the first time).

Related: custom content for Welcome page