Make the width/height in the SVG metadata box be in the SVG's units

Make the width/height in the SVG metadata box be in the SVG's units

This was suggested by a user at commons. The width/height in px is really
a MediaWiki thingy needed for rendering the image, from a metadata prespective
its the original units (aka the image should be 10cm, etc) that is
the information associated with the file.

The dimensions in pixels is still specified in the long description
of the image (the subtitle directly under the image).

Note: After gerrit change Ic58efbf2 is merged, this will cause no width or height
to be displayed for svg images with cached metadata, until they
get purged. I think that's perfectly ok as the width is
available elsewhere on the page.

p.s. I added one semi-unrelated code comment in SVGMetadataExtractor.

patchset 2,3: Fix test (hopefully, how come getting an up to date
version of PHPUnit is so difficult)
patchset 4: rebase
patchset 5: fix my accidental remove of a comment

Change-Id: I312fbd1c935a0095644c3b63c4929632c6f6e387