Allow displaying SVG images in documentation

Allow displaying SVG images in documentation

PostgreSQL 12 adds SVG images in the documentation, so the website
should be able to display them.

The images themselves were already loaded by the docsloader, but the
regexps in the URL would block them from being seen. Fix this by
creating a separate function for the SVGs, since we also don't want to
render them inside teh templates.

This new view must also be tagged with @allow_frames, since the browser
considers the <object> tag used to be a subframe. Without this, they
would be blocked from viewing even on our own site.