FEATURE: add China holidays and region

[devhaozi]

This PR attempts to add China holidays and region definitions to discourse-calendar plugin.

I am a beginner in Ruby and discourse plugin development, if there is any problem please help me fix it. Thank you so much.

[jjaffeux]

@devhaozi hi, sorry for delay, do you still want to contribute this?

[devhaozi]

@devhaozi hi, sorry for delay, do you still want to contribute this?

Yes

[davidtaylorhq]

Moving to core repo