Rework the handling of clientStrings

Summary: Looking at the profile of page load, there is frequently a long pause (up to a second) while clientStrings get loaded. That's idiotic -- do something about it.

Unfortunately, preload doesn't seem to work properly for clientStrings, presumably because it doesn't fit any of the "as" types allowed in link preload. When I try, it just double-fetches.

Given that, and the fact that parsing clientStrings is still fairly slow even after my improvements to shocon, we should seriously consider dropping the client-side handling of it entirely. Instead, we might pre-parse it server-side into a more-efficient data structure. The ideal might be to translate it into conventional JSON, and embed it directly into client.html, so that the client can just start using it. That could speed up load time considerably.