Create a legacy iframe and update styling on the default

After using WYMeditor with lots of users over several years, there are many small iframe styling tweaks that improve usability (without really any downside). In order to bring those to other WYMeditor users, we need to change the default iframe.

Some folks might be reluctant to change, though, so let's move the current iframe stuff to a `legacy` iframe that they can use. Document that option and document that support for that iframe is deprecated and it will eventually be removed from the core editor.