* feat(backend): add isSensitive to Channel
* feat(backend): support isSensitive in channel endpoints
* feat(frontend/channel-editor): support isSensitive in create/edit channel page
* feat(frontend/channel): show sensitive indicator for sensitive channels
* docs(changelog): add チャンネルをセンシティブ指定できるようになりました
* chore: license header for each file
* chore: add isSensitive of channel to Note object
