Basic setup for multi-lingual version

[jan-niestadt]

This is a very basic setup for translating the app using vue-i18n.

Default message files are in src/frontend/src/locales, e.g. en.json.

Per-corpus override message files can be placed in $corpusInterfaceDataDir/static/locales with the same name, e.g. en.json.

There's no locale switching widget yet, but you can test it using vueRoot._i18n.locale = 'nl;` in the browser's dev console.

TODO:

• switching languages prints a warning on the console that the messages for the new locale can't be found, until they are actually loaded
• if no override file is defined, a 404 is visible in the console