Convert native GHC build of codeworld-api to use jsaddle instead of blank-canvas

Currently, the native GHC build of codeworld-api depends on blank-canvas, which has very limited support for JavaScript events and expressions. Instead, I'd like for it to use jsaddle, which can run a much wider set of JavaScript expressions on the client. This should allow a lot more code sharing between the two builds.

Some problems this will solve:

1. The `codeWorldLogo` picture will work in native mode.
2. The `TextEntry` event constructor will work in native mode.
3. Multiuser (`groupActivityOf`) programs can work in native mode. This will require that the host process link in code for the relay server, as well.