Add template examples to the template step interface

A request from garygaughan for the template step

When adding a template step the user is presented with little guidance, i.e. there's a selector for the template type but there's no examples.

It'd be nicer if the template step could have placeholder text with a simple example specific to the selected template language. If the user doesn't make changes within the template editor and instead changes the template language, it'd be ideal if the placeholder example is updated to reflect the selected template language.

If the user has made edits to the contents of the template editor, then this placeholder/example functionality should be disabled to preserve the user's input.