Add SVG version of user icon in Vector personal portlet

Add SVG version of user icon in Vector personal portlet

An SVG version of the user icon is provided only for browsers supporting
SVG. To ensure browser compatibility, the SVG version is provided in a
two-layer background where the first layer is a transparent gradient.

The fact that browsers supporting CSS gradients are a strict subset of
those supporting SVG guarantees an appropriate fallback.
Embedding is used to avoid extra http requests.

Bug: 35341
Change-Id: I914da0649459744ccca9e1a78e9f48fe66e1a77f