Both src and srcset images being downloaded

[reefdog]

Open the single test page and check the Network tab of the inspector. Expected behavior of <img src="…" srcset="…"> is that the browser only fetches the most appropriate one; instead, both images download.

[reefdog]

Two-part problem:

• [x] For at least some browsers, when srcset exists, src is entirely ignored, even if the minimum width conditions described in srcset aren't met. That seems odd, but whatever. Regurgitating the src image in srcset, while verbose, fixes it.
• [x] The image prefetcher used to calculate note layout always prefetches the normal (700px) image. This is advantageous since the notes themselves are pixel-positioned on the normal image dimensions, and it was also inert back before we added srcset. Now with srcset possibly preferring a large image, and with dimension or aspect ratio potentially available to us via the private API, we should try to avoid this as it means we're always fetching the normal image, too, even when we won't be displaying it.