Display SVGs in target language

Display SVGs in target language

Previously, they were always displayed in defult language unless
forced explicitly in wikitext, e.g. [[File:Foo.svg|lang=ru]].
This change adds a feature flag that would enable always trying to
display in page language.

* If enabled, Parser will pass a new parameter - 'pagelang' - to
  the media handler.
* SvgHandler uses page language when determining what language to
  render the image in.
* 'pagelang' can always be overridden by 'lang'.
* If no translation in page language is available, the default
  language (English) will be used for thumbnail URLs, to prevent
  cluttering media storage and HTTP caches with useless copies.

Performance: this requires accessing image's metadata during parsing.
My testing indicates there were no code path where this wasn't the
case already, so no performance hit is expected, however we should
still keep an eye on page save performance.

Bug: T205040
Change-Id: I348840ef405e1370cc0c17d69051bce30153c9c0