UploadBase: Permit SVG files with broken namespace definition (Inkscape bug)

UploadBase: Permit SVG files with broken namespace definition (Inkscape bug)

Inkscape mangles namespace definitions created by Adobe Illustrator
(apparently it can't parse custom entities or something, maybe just
in 'xmlns' attributes). These files are still valid SVG, and not
a security issue (although Illustrator probably won't like them),
so it's okay to allow them.

Added tests with some example files.

* buggynamespace-original.svg
  File generated by Illustrator (edited by hand to reduce filesize).
  Based on <https://commons.wikimedia.org/w/?curid=16495597>.

* buggynamespace-okay.svg
  The original file, opened and saved in Inkscape (no other changes).

* buggynamespace-okay2.svg
  The original file, opened and saved in Inkscape twice.

* buggynamespace-bad.svg
  The original file, edited by hand to remove custom entities.
  This is not valid XML and should be rejected (although it's valid
  when parsed as HTML, and some image viewers might display it).

* buggynamespace-evilhtml.svg
  An SVG file using an entity declared namespace for a namespace
  we want to ban. Based on buggynamespace-original.svg.

Bug: T144827
Change-Id: I0eb9766cab86a58d729f10033c64f57d2076d917