Removed Jed from the headless build and instead let `__` call sprintf.
It's now up to downstream users for `@converse/headless` to decide
whether they want i18n support for the few translatable strings in that package
and to implement it themselves.
After some back-and-forth, I think it's still better to keep markup in
templates (instead of having them directly inside the components)
becaues it makes it easier for people to modify Converse.js (at the
expense of some developer ergonomics).
