Restructure frontend to use re-frame

This doesn't actually create any functionality, but can be run properly.

To run the project with re-frame, run "shadow-cljs watch app" in the project folder. In a separate terminal/repl start the clem app normally.

It currently only displays a "hello world" proof of concept.

I'm pushing this up as a merge request early, in order to get feedback on the entire process, as further steps will build on what I do now. I would appreciate your thoughts on what's going on so far.