fix(localization): pybabel doesn't extract plural forms from frontend

[artemonsh]

SUMMARY

Currently there is an issue with the plural forms on the frontend. At first, let's take a look at this code:

superset/superset-frontend/src/explore/components/controls/DndColumnSelectControl/DndColumnSelect.tsx

Lines 206 to 210 in 7d8fff8

	tn(
	'Drop a column here or click',
	'Drop columns here or click',
	multi ? 2 : 1,
	)

It is expected that while extracting these strings into .po file using babel-update.sh they should look like this:

msgid "Drop a column here or click"
msgid_plural "Drop columns here or click"
msgstr[0] ""
msgstr[1] ""

or this:

msgid "Drop a column here or click"
msgid_plural "Drop columns here or click"
msgstr[0] ""
msgstr[1] ""
msgstr[2] ""

depending on the number of plural forms in the language. For instance, English and French have 2 plural forms and Russian has 3.

But in reality none of the .po files contain plural forms. For instance, .po file for French translations, which has 2 plural forms

superset/superset/translations/fr/LC_MESSAGES/messages.po

Line 25 in 7d8fff8

"Plural-Forms: nplurals=2; plural=(n > 1)\n"

does not contain the desired syntax for translating both of forms:

superset/superset/translations/fr/LC_MESSAGES/messages.po

Lines 4873 to 4874 in 7d8fff8

	msgid "Drop a column here or click"
	msgstr "Supprimer une colonne ici ou cliquer"

The solution to this issue is simple and is offered in this PR. This is the only official source I have found to prove my changes, but I found same solutions on some russian websites.

TESTING INSTRUCTIONS

Go to the root directory of superset and run ./scripts/babel-update.sh.

ADDITIONAL INFORMATION

• Has associated issue: closes Plural forms are not compiled from .tsx files to messages.pot for translation #21701
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[villebro]

LGTM and confirmed to work as expected.

[villebro]

Bycatch - we probably don't need -k tct here as I don't believe we even have such a translator function, but I suppose it doesn't matter that much.