Inject base url to tracker at runtime (#78)

config/config.exs

config/test.exs

lib/plausible/application.ex

lib/plausible_web/controllers/tracker_controller.ex

lib/plausible_web/router.ex

priv/tracker/js/analytics.js

priv/tracker/js/p.js

priv/tracker/js/plausible.js

tracker/compile.js

tracker/src/p.js

tracker/src/plausible.js