Use happy holidays instead of Merry Christmas

Use happy holidays instead of Merry Christmas
Missing the translations though.

[fixes #634]

maybe @kimroen @JoahG @steverob @Jylhis @romainberger @endelwar
can help with the translation of:
- "Happy Holidays"
- "Happy Holidays and see you all next year when we do it all again!"

For now I am leaving the file without them, people could PR that individually, otherwise will fallback to the english version.

@despo @andrew thoughts?