displaytitle: reject some CSS if $wgRestrictDisplayTitle set

displaytitle: reject some CSS if $wgRestrictDisplayTitle set

$wgRestrictDisplayTitle is intended to make it possible to simply
copy-and-paste the title text even if it requires some styling like
subscript or superscript. Using a <span style="display: none;" />
broke that expectation, as the text hidden in such way becomes
completely invisible and unselectable. This patch rejects such styles.

Also disallowed 'user-select' and 'visibility', since they both
prevent the user from selecting and/or copying the text as well.

Minor changes in Sanitizer:
* checkCss() was made to pass through values which consist of nothing
  but a single comment, to allow this rejection to display some sort
  of a notification to the user.
* encodeTagAttributes() was added as a counterpart to
  decodeTagAttributes(), pulling some code out of fixTagAttributes().

Bug: 26547
Change-Id: Ie162535b6bcbebce4ee69f6dcc1957ccccc3c672