Markdown table in code mirror

[atbah]

Closes #2753

[saracarl]

So this works well, even with other markup buttons turned on:

However, this is an "advanced" sort of thing that would easily confuse transcribers, so I think we want to make it part of the editor buttons a project owner has to turn on. (For example here: http://localhost:3000/umdtranscription/david-c-driskell-papers/edit/tasks#edit-buttons)

Let's call it "Table" with the text "Inserts a markdown table of transcriber specified rows and columns"
Put it after superscript in the list.
Instead of trying to describe it after the checkbox, let's just say "Documentation" with a link to https://content.fromthepage.com/project-owner-documentation/table-encoding/