js: Make lang code handling more robust in config

Differing internal representations of two-level language
codes, e.g. pt_BR/pt_PT or zh_CN/zh_TW, would lead to
inconsistencies when combined with overrides of i18n strings
for templating.

E.g. setting data-isso-lang=pt_BR and
data-isso-postbox-text-text-PT_BR would result in an
internal representation of data-isso-postbox-text-text-pt-br
(note: lowercase) and thus user-supplied i18n strings not
being applied.

To make this handling more robust, convert everything into
lowercase and substitute underscores for dashes.

Fixes isso-comments#982

isso/js/app/config.js

@@ -17,9 +17,21 @@ for (var i = 0; i < js.length; i++) {
         var attr = js[i].attributes[j];
         if (/^data-isso-/.test(attr.name)) {
             try {
-                config[attr.name.substring(10)] = JSON.parse(attr.value);
+                // Normalize underscores to dashes so that language-specific
+                // strings can be caught better later on,
+                // e.g. data-isso-postbox-text-text-PT_BR becomes
+                // postbox-text-text-pt-br.
+                // Also note that attr.name only gives lowercase strings as per
+                // HTML spec, e.g. data-isso-FOO-Bar becomes foo-bar, but since
+                // the test environment's jest-environment-jsdom seemingly does
+                // not follow that convention, convert to lowercase here anyway.
+                config[attr.name.substring(10)
+                       .replace(/_/g, '-')
+                       .toLowerCase()] = JSON.parse(attr.value);
             } catch (ex) {
-                config[attr.name.substring(10)] = attr.value;
+                config[attr.name.substring(10)
+                       .replace(/_/g, '-')
+                       .toLowerCase()] = attr.value;
             }
         }
     }

isso/js/app/i18n.js

@@ -133,7 +133,10 @@ if (!plural || !translations) {
 }
 
 var translate = function(msgid) {
-    return config[msgid + '-text-' + lang] ||
+    // Need to convert the language string to lowercase because data-isso-*
+    // attributes are automatically cast to lowercase as per HTML spec
+    // https://stackoverflow.com/questions/36176474/camel-case-in-html-tag-attributes-and-jquery-doesnt-work-why
+    return config[msgid + '-text-' + lang.toLowerCase()] ||
       translations[msgid] ||
       catalogue.en[msgid] ||
       "[?" + msgid + "]";