Add option to hide the language selector (#1106)

* Added option to hide the language selector in the header
* Added `hide_language_selector` Switch to BasicsPage
* Added `hideLanguageSelector` property to ForumSerializer
* Apparently fixed the "Add Extension" button locale.... someone must not have compiled their changes :P

* Changed hideLanguageSelector (and such) to showLanguageSelector

* Change `core.admin.basics.show_language_selector_heading` to be `_label`

* Change showLanguageSelector in ForumSerializer to be boolean, default: true

* Ooops! Remove console.log 🤦‍♂️

js/forum/dist/app.js

@@ -23215,7 +23215,7 @@ System.register('flarum/components/HeaderSecondary', ['flarum/Component', 'flaru
 
             items.add('search', app.search.render(), 30);
 
-            if (Object.keys(app.data.locales).length > 1) {
+            if (app.forum.attribute("showLanguageSelector") && Object.keys(app.data.locales).length > 1) {
               var locales = [];
 
               var _loop = function _loop(locale) {