Allow upload of SVG images

It should be possible to upload SVG images into the CMS. This are vector images stored as XML, and can be scaled freely by the browser.

Each SVG image also stores a default width and height in the 'width' and 'height' attributes of the root 'svg' tag in the .svg file. The CMS should create different scaled variants by simply changing the 'width' and 'height' attributes in each stored variant.

A nice example .svg file can be found at http://ariya.github.io/svg/tiger.svg.