redlinks: Move contents instead of serialising/reparsing html

Follows-up b0ca6a299f.

No need to serialise the node to html, then parse it again, and
destroy the original. If there were reason to re-create the
contents, a clone() would be more appropiate, but even that's
not needed here. We can just move the contents directly.

Also:
* Fix invalid html in '<span class="new">'.
* Use replace instead of insert/remove to avoid extra reflows.
  (Mobile performance.) Previously it forced two reflows. One
  to render the elements next to each other, one to remove the
  first element. Now it will actually not reflow at all in most
  cases (not even once) as the width of the text is the same
  regardless of color (it merely changes from red to black).

Change-Id: I9d850fba0ecb7498d684c4416f873f31556ab229
Merge "redlinks: Move contents instead of serialising/reparsing html"