feat(i18n): bring the Slovak translation up to date with dev

Translates the keys dev gained since the branch was cut, the help center
above all, and drops the six keys dev no longer has. Registers sk
everywhere a language is listed: notification emails, the rate-limit
message, the placeholder check and the wiki. The two tests that counted
the languages by hand now derive the count from SUPPORTED_LANGUAGES.

shared/src/i18n/externalNotifications/index.ts

@@ -16,6 +16,7 @@ import ko from '../ko/externalNotifications';
 import nl from '../nl/externalNotifications';
 import pl from '../pl/externalNotifications';
 import ru from '../ru/externalNotifications';
+import sk from '../sk/externalNotifications';
 import sv from '../sv/externalNotifications';
 import tr from '../tr/externalNotifications';
 import uk from '../uk/externalNotifications';
@@ -41,6 +42,7 @@ const LOCALES = {
   nl,
   br,
   cs,
+  sk,
   pl,
   ru,
   zh,