newsticker: Handle embedded (x)html in atom feeds.

* lisp/net/newst-backend.el (newsticker--parse-atom-1.0): Handle
embedded (x)html in summary node.
(eww-process-text-input): Try to keep track of the size more reliably.
* net/eww.el (eww-restore-history): Bind `inhibit-modification-hooks'

instead of `after-change-functions'.