Handle SVG-based user avatars better

Currently, if Whim accepts a webmention from someone whose avatar is stored on the internet as an SVG file, it doesn't do a very good job presenting it.

(I think this is because Whim doesn't give browsers any clues about image file formats via either filename-extensions not content-type headers, and SVGs seem to be outside the purview of most browser's "well, i'll try to guess anyway" powers.)