Implement page status indicators

Page status indicators are icons (or short text snippets) usually
displayed in the top-right corner of the page, outside of the main
content. Basically, <indicator name="foo">[[File:Foo.svg|20px]]</indicator>
may be used on a page to place the icon in the indicator area. They
are also known as top icons, page icons, heading icons or title icons.

I found the discussion on bug 23796 highly illuminating. I suggest
that everyone read it before suggesting different design choices.

I spent some time with a thesaurus pondering the name. "Emblems" and
"badges" were also considered, but the former has a much more limited
meaning and the latter is already taken by Wikidata, with a similar
but subtly different feature set. I am not aware of any naming
conflicts ;) besides new talk page message "indicator" (used by core
and Echo in some documents) and OOjs UI indicators (tiny icons like
the arrow on a dropdown form element), which shouldn't be confusing.

Potential use cases include:
* "Lock" indicators for page protection levels
* Featured/good article indicators
* Redirect shortcuts display ("WP:VPT")
* Links to help/manual for special pages
* Coordinates?… or globe icon for inline pop-up maps

Design features:
* Skin-customizable. Skins can fully control where and how indicators
  are shown, or may just do <?php echo $this->getIndicators(); ?> to
  output the default structure. By default they are not shown at all.
* Extension-customizable. Extensions can call ParserOutput::addIndicator()
  to insert an indicator from one of the numerous parser hooks.
* Wiki-customizable. In addition to just using the parser functions,
  on-wiki styles and scripts can use the provided classes and ids
  (.mw-indicator, #mw-indicator-<name>) to customize their display.

Design limitations:
* Every indicator must have a unique identifier (name). It's not
  possible to create arrays, or to have several indicators with the
  same name. In case of duplicates, the latest occurrence of the
  parser function wins.
* Indicators are displayed ordered by their names (and not occurrence
  order). This ensures consistency across pages and provides a simple
  means of ordering or grouping them.
* Indicators are not stored, tracked or accessible outside of
  ParserOutput (in particular they're not in the page_props table).
  They are intended to merely reflect the content or metadata that is
  already present on the page, and not be data themselves. If you ever
  think you need to list pages with a given status indicator, instead
  figure out what it means and use the appropriate tracking category,
  special page report, already existing page_prop, or other means.

Corresponding patch in Vector: I90a8ae15ac8275d084ea5f47b6b2684d5e6c7412.
I'll implement support in the other three skins included in the tarball
and document it on mediawiki.org after this is merged.

Bug: 23796
Change-Id: I2389ff9a5332a2b1d033eb75f0946e5241cfaaf4