added ability to switch back and forth between html editor and regular item authoring

- imported items have html editor by default, can be switched to
regular authoring (source and media will be blank, but returning to
html mode will restore original html)
- regular items can be switched to html editor mode, if no previous
html is saved, the source and media (images only, editor doesnt work
with video or audio) are saved as html and the editor is loaded with
them in it, switching back to reuglar authoring restores the original
source and media