New options for the 'data type' drop-down

I've really appreciated some of the templates in the 'data type' drop-down recently, particularly "long text string" and "list of short strings". I'm sure there are more common patterns that we can make templates for, to avoid writing JME code.

This issue collects ideas for new options.

* "long plain text string" - a codemirror editor for plain text, which can be marked as a template so it's wrapped in `safe()`. The current "long text string" should be renamed to "rich text" or "formatted text".
* "Variable group as list/dictionary" - see #503

Start by reviewing the existing “data type” drop-down templates, especially “long text string” and “list of short strings.” Consider the proposed plain-text CodeMirror editor, template wrapping with safe(), renaming the current rich-text option, and the related variable-group idea in #503. Done means the selected new options and naming are agreed and implemented.

Written by the indexing model from the issue text.