Additional check to ensure temporary compose files are deleted on logout
Initial work in allowing pasting of data (images) into HTML editor

For now, a drag/paste will simply add the data as an attachment.  Still
need to (among other things):

  - Add HTML tag in editor to link to attachment data (compose attach
    preview)
  - Indicate that this data should not be a true attachment but rather
    data in a multipart/related part.

This is only ever going to work in dynamic mode.  This is never going to
be available in basic mode.