Use CKEditor instead of Markdown/Pagedown for email content

  - Markdown widget is deprecated
  - Clients want HTML, not markdown

For plaintext email clients, we use html2text to convert the HTML to
markdown. This patch is backwards compatible, because it converts all
the existing markdown to HTML in the migration.
Add an 'include_form_data' option to EmailSuccessHandlers

This allows the data from the form submission to be included in the
success email.
Fix error when `to_ident` is blank in a EmailUserHandler

We need to handle the case where the form is submitted before the
EmailUserHandler is ready.