Related to AAP-7379,
part of AAH-2045, AAP-5615

Problem Description:

Platform ui will be using i18next, not lingui, as the javascript l10n library.

Proposed Solution:

We switch to i18next.

This involves:

• switch the library to `i18next`:
  • `src/l10n.ts` logic to load the right translation on demand (keep `moment` logic)
  • `gettext:extract` and `gettext:compile`
  • all imports of `<Trans>` and `t`, and any uses that need to be changed
  • specifically any plural rules will need the numeric variable to be called `count`
• dropp all uses of `@lingui/*` npm packages,
• convert existing `locale/.js` to the new (json) format, drop `locale/.po` (rename the variable for plurals to count here as well)
• notify Christian Adams to switch the format to json on the translators side (and mention if there were any changes to the extract&compile interface, or any path changes)
• update lint-po to handle the new format