PGFTikZ extension: initial import

Tag extension (<PGFTIkZ> tag) used to input and render PGF/TikZ code.
The code is placed in a temporary LaTeX file, compiled using the
following sequence of commands:
-latex (generates a .dvi from the generate .tex file)
-dvips (generates a .eps file from the .dvi file)
-epstool (extracts a minimal bounding box from the .eps file)
-convert (ImageMagick converts to the final file type)
The generated image is uploaded to the wiki, and the source used to
generate it is saved in the image's wikipage.  When parsing the tag,
the source code is tested against the saved code to skip compilation
if it is not needed (see mediawiki.org/wiki/Extension:PGFTikZ).
Update after review.

Change-Id: I261a697d687c8bde84dfd4d3d5c3c28962ea6b65
Consistency tweaks

* Use single quotes in i18n file
* Image -> File
* Add FIXME for message documentation
* Fix extension type

Change-Id: I0ed44a981862df277f7396fbc79b45bbf59b156b
Migrate to JSON i18n

Procedure per https://www.mediawiki.org/wiki/Manual:GenerateJsonI18n.php
with shim.

Change-Id: I9506369d5bb65fcfd39b403d7198ba8602294be2