Allow text formats to be cloned.

Cloning and tweaking instead of having to start from scratch is much easier for people.

This may be a step towards solving some of the UX issues discussed in #1519 too. Site admins seem to be using the "Full HTML" format as a go-to solution when content editors demand more. This is OK in general, but it creates false expectations that people will get a WYSIWYG etc.

If we provided an easy way for admins to clone the "Filtered HTML" format, then they would do that, and add buttons + allow additional HTML tags as required (which is what they should be doing).