Decodes the entities rich text commonly holds: &, <, >, ", ' and
(as a space). Decodes once, so &lt; becomes <. Borrows the input when it
has no &.
A URL path segment made of a title: its letters and digits, of any script, in lowercase, with
each run of other characters as one - and none at either end. Empty when the title has no
letters or digits, so the caller picks a fallback.