validate and remove conflict data when copy event to new draft

[dotanthanh]

Fix for issue #311 and #279. Notices:

• Now before copy from an existing event to a new draft, the existing event data will be validated.
• After validating, all invalid fields will be emptied based on their types: array -> [], object -> {}, string -> '' ...etc
• Fields with tricky types (if there is any) like null, undefined, boolean... should be left unchanged.

There is no validation for location and keywords in UI. Maybe these should to a later separate issue.

Signed-off-by: Thanh Do [email protected]

[Rikuoja]

This is a good fix, just check the comments below :)