feat: map_locale_display_names(): skip (hide) unusable locales

We can't iterate directly over USABLE_LOCALES: Set(Locale) because
Locales aren't sortable.  Instead, we parse config.SUPPORTED_LOCALES in
order and skip those that aren't in USABLE_LOCALES.

securedrop/i18n.py

@@ -179,11 +179,17 @@ def map_locale_display_names(config: SDConfig) -> None:
     """
     language_locale_counts = collections.defaultdict(int)  # type: Dict[str, int]
     for l in sorted(config.SUPPORTED_LOCALES):
+        if Locale.parse(l) not in USABLE_LOCALES:
+            continue
+
         locale = RequestLocaleInfo(l)
         language_locale_counts[locale.language] += 1
 
     locale_map = collections.OrderedDict()
     for l in sorted(config.SUPPORTED_LOCALES):
+        if Locale.parse(l) not in USABLE_LOCALES:
+            continue
+
         locale = RequestLocaleInfo(l)
         if language_locale_counts[locale.language] > 1:
             locale.use_display_name = True