Old upstream translations overwrite newer upstream translations in Ubuntu

Launchpad generally prefers upstream translations over Ubuntu translations. So much that it blindly sets the upstream translation as current if it was not already overridden.

However, with translations sharing, this causes a problem: we can have older translations re-imported on an older series (eg. in Natty with a bugfix package), and these translations would now override more recent translations (in eg. Oneiric). Until newer Oneiric package is provided again, older translations will be used.

Translations preference policy should check the date when replacing existing upstream translation with the one being imported.

Potential examples given in bug #818230 by Jiri Grönroos:

https://translations.launchpad.net/ubuntu/oneiric/+source/bluedevil/+pots/bluedevil/fi/+translate?batch=10&show=all&search=No+supported+services+found
Located in monolithic/monolithic.cpp:283 monolithic/monolithic.cpp:575
(this translation is different from what I translated here on Launchpad)

https://translations.launchpad.net/ubuntu/oneiric/+source/file-roller/+pots/file-roller/fi/+translate?batch=10&show=all&search=Find...
Located in ../src/ui.h:127
(this translation is the same that I translated here on Launchpad)

https://translations.launchpad.net/ubuntu/oneiric/+source/eog/+pots/eog/fi/+translate?batch=10&show=all&search=External+program+to+use+for+editing+images
Located in ../data/org.gnome.eog.gschema.xml.in.in.h:10
(this translation is different from what I translated here on Launchpad)

https://translations.launchpad.net/ubuntu/oneiric/+source/seahorse/+pots/seahorse/fi/+translate?batch=10&show=all&search=This+was+the+third+time+you+entered+a+wrong+password.+Please+try+again.
Located in ../pgp/seahorse-gpgme-key-op.c:300
(this translation is different from what I translated here on Launchpad)