Allows disabling of timestamps when importing posts

fixes #1696

- this is a temp workaround until full fledged support
is added directly to bookshelfjs
- when importing we use the import json blob’s timestamps
as the value that’s set in the DB 
- added tests for this change
Merge pull request #1759 from hswolff/fix-import-updated-at

Allows disabling of timestamps when importing posts