Add the ability to translate submissions (redux) (#4219)

This builds on the #4134 PR that initially introduced machine
translations into Hypha. This isolates the translation behavior; putting
pip dependencies in a separate `requirements-translate.txt` and will not
attempt any translate imports unless the setting for it is true.

Other small changes are also a full docs page explaining how to install
language packages & changing the setting once again from
`SUBMISSION_TRANSLATIONS_ENABLED` to `APPLICATION_TRANSLATIONS_ENABLED`
to reflect the system wide shift away from submission terminology.