ExpandedScrollingColumn is on top of the AppHeader and blocks presses on the left half of the language switch button. This commit moves the AppHeader to the top of the stack.
- add LocaleButton to intro and menu, toasted placeholder in app_scaffold
- add localeLogic.isEnglish helper, used in _results_grid and _scrolling_content
- stubbed in button for testing locale switching, needs design polish and direction on whether the button is available at all times or just in menu
- tested and fixed switching on the fly for wonder views, intro, timeline, artifacts, and menu