[#1029] Allow to override's post timestamp.
[#1029] Allow to override's post timestamp.

Required for migration. Should be done on creation, so proper slugs were
generated.

lym/allura-git,leotrubach/sourceforge-allura,Bitergia/allura,heiths/allura,leotrubach/sourceforge-allura,apache/incubator-allura,lym/allura-git,leotrubach/sourceforge-allura,lym/allura-git,lym/allura-git,apache/allura,heiths/allura,leotrubach/sourceforge-allura,heiths/allura,heiths/allura,apache/incubator-allura,Bitergia/allura,apache/allura,Bitergia/allura,apache/allura,Bitergia/allura,apache/allura,apache/incubator-allura,apache/incubator-allura,heiths/allura,apache/allura,lym/allura-git,Bitergia/allura