Fix i18n dashboard language strings and allow customizing of permalink slugs

[brianhogg]

Description

Various i18n fixes:

• Menu options in the WordPress admin should now show in the user's language
• Loads the text domain later to allow Loco Translate to change where translations are loaded from
• Set the slugs for courses, memberships and other core taxonomies to the current site language, and provide options to change them from Settings > Permalinks. This avoids slugs inadvertently changing if a user with access to flush the permalinks has a different language set than the site language.
• LifterLMS blocks in the Course editor and elsewhere now load translations, and are translatable via Loco Translate

Fixes #2324 Fixes #2429 Fixes #2525

How has this been tested?

Manually. The current E2E doesn't permit tests before LifterLMS has been loaded, and switching languages was flaky.

Screenshots

Block translations showing Menu options and notices showing in the user language rather than the site language (French in this case) Settings > Permalink options to modify LifterLMS permalink slugs. Shows field added via `lifterlms-assignments` add-on.

Checklist:

• [x] This PR requires and contains at least one changelog file. <!-- To create a changelog yml file: npm run dev changelog add -- -i and follow the prompt. See also: https://github.com/gocodebox/lifterlms/blob/trunk/packages/dev/README.md#changelog-add -->
• [x] My code has been tested.
• [x] My code passes all existing automated tests.
• [x] My code follows the LifterLMS Coding & Documentation Standards.

[github-actions[bot]]

Coverage report

[!CAUTION] An unexpected error occurred. For more details, check console

Error: The process '/usr/bin/git' failed with exit code 128

St.:grey_question:	Category	Percentage	Covered / Total
🟡	Statements	77.18%	328/425
🟡	Branches	68.45%	141/206
🟢	Functions	82.93%	68/82
🟡	Lines	76.68%	319/416

Test suite run success

169 tests passing in 22 suites.

Report generated by 🧪jest coverage report action from e023c83083389983ba85455295baa5c9857cdaea