Django command to debug i18n translations

[OmarIthawi]

This command makes it easier to debug translations without the need to use the GUI. This should remove the most annoying step when debugging i18n issues for missing translations.

I'll update our Runbooks and this is a good thing to be contributed to the Open edX community.

Related: Problem with xblock-utils:ProxyTransNode.merge_translation method

The merge_translation method isn't cleaning up after the merge and leaves stray translations in the following way:

• ✔️ Calling `i18n.helpers.translate('fr_CA', 'Thank you.') returns the untranslated 'Thank you.'
• ✔️ Calling `i18n.helpers.xblock_translate('fr_CA', 'Thank you.') returns 'Merci !'
• ❌ Calling i18n.helpers.translate('fr_CA', 'Thank you.') again returns the the translated 'Merci !' which means that the merge_translation didn't clean up after itself

Workaround: Call translate then xblock_translate in the tahoe_translate command.

Long term fix: Work with upstream to fix the cleanup process around translation.activate(...) as documented in #954

[coveralls]

Coverage increased (+0.03%) to 42.025% when pulling 888a693 on i18n_debug into e7db32d on main.