Refactor i18n implementation

One can now use $this->translate(msg) in a view or controller without the
need to pass the module domain (web&cli). Forms still need to be built
with _t(msg) and _mt(dom, msg).

refs #5533
Refactor translation cli commands and implement po-file compilation

refs #5533
Add language selection to general config and preference forms

refs #5533
Merge branch 'feature/translation-utils-5533'

resolves #5533