Extract admin strings

Adds app.trans calls for strings used by the admin UI.
- Strings for AddExtensionModal.js not included.
- Corresponding YAML will be sent later w/ more extracted strings.

js/admin/src/components/SessionDropdown.js

@@ -43,7 +43,7 @@ export default class SessionDropdown extends Dropdown {
     items.add('logOut',
       Button.component({
         icon: 'sign-out',
-        children: app.trans('core.log_out'),
+        children: app.trans('core.admin.header_log_out_button'),
         onclick: app.session.logout.bind(app.session)
       }),
       -100