Manage serving picts in the server better

[benknoble]

Adding new picts to the server seems like a mess, and the conversion from string to symbol seems very unnecessary. Let's tidy that up.

[benknoble]

I rarely need to add new picts: I probably could have done the element pics differently, but I like the way they are handled as routes for now (everything else is direct SVG content in the response, which means bigger responses or SSEs for simpler code).