Cleanup HtmlWriter API

Summary:
- merge Interface::HtmlWriter and HtmlWriter
- remove queue() and flush(), which with the single synchronous buffer
  send to WebEngine have no meanings anymore
- remove the unused CSS argument to begin()
- remove TeeHtmlWriter, its debugging use has been superseded by the much
  more convenient direct access to the HTML source code in the mail source
  view

This prepares the next step of making HtmlWriter conceptually a QIODevice,
so we can use it directly with Grantlee's streaming API.

Reviewers: knauss

Reviewed By: knauss

Subscribers: #kde_pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D8083