web: skeleton code with most of the stuff from osms-web

- The new `tspl-web` crate is kind of a copypasta from `osms-web`,
  and kind of not.
- All the templating stuff, templates, and CSS/JS are straight
  copies.
- The rest of the logic is somewhat different (read: better!).
- It doesn't actually do anything past display the index page
  and error out a lot, though.