Upgrade to vue-i18n 8.27.1

Note: at  the time of writing we can't upgrade because the webjar most recent version is 8.15.5.

See

https://github.com/kazupon/vue-i18n/releases/tag/v8.25.1
https://github.com/kazupon/vue-i18n/releases/tag/v8.26.0
https://github.com/kazupon/vue-i18n/releases/tag/v8.26.1
https://github.com/kazupon/vue-i18n/releases/tag/v8.26.2
https://github.com/kazupon/vue-i18n/releases/tag/v8.26.3
https://github.com/kazupon/vue-i18n/releases/tag/v8.26.4
https://github.com/kazupon/vue-i18n/releases/tag/v8.26.5
https://github.com/kazupon/vue-i18n/releases/tag/v8.26.6
https://github.com/kazupon/vue-i18n/releases/tag/v8.26.7
https://github.com/kazupon/vue-i18n/releases/tag/v8.26.8
https://github.com/kazupon/vue-i18n/releases/tag/v8.27.0
https://github.com/kazupon/vue-i18n/releases/tag/v8.27.1