Add language selector

[sruenwg]

Closes #432, closes #536.

What this PR solves / how to test:

Changes

• Added language selector to drawer
• Configured middleware so that the user is directed to the first valid locale found in: URL locale > NEXT_LOCALE cookie > Accept-Language header (e.g. en-GB would send the user to /en) > default locale (ja).

Tests

• Changing the locale with the language selector adds a new entry to the browser history with the selected locale
• The NEXT_LOCALE cookie and the language shown in the language selector are always in sync with the URL locale
  • Browser forward/back updates all three items correctly
• First visit (empty cookies) to / redirects to:
  • /en/schedule/all if the browser locale (the Accept-Language header sent) has en* set to a higher priority than ja*
  • /ja/schedule/all otherwise
• Manually entering a URL with a locale overrides any previous cookie set

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[sugar-cat7]

Wow, thank you! It's perfect!!