Fix SSR / cleanup server index

[adriaanwm]

Currently ssr is not working for dynamic paths like users/:id because of https://github.com/inputlogic/web-starter/blob/master/server/index.js#L37-L50

Also, going to reorder https://github.com/inputlogic/web-starter/blob/master/server/index.js#L134-L136 so that we only try rendering preact if the url doesn't match a file