Allow locking translations

[Aaron Bentley]

When I start translating untranslated items, I would like to be able to temporarily "lock" the Launchpad translation page, in order to notify other translators that they don't need to work redundantly on the same items.

Similarly, when I filter a project for untranslated items, I would like these locked items to be excluded too, so I can work on the items available for translations, not the ones currently being translated by someone else.

The implementation could be a "Lock translations" button for the whole page, or maybe a more configurable "Lock" column, with one generic checkbox for the whole page, and individual checkboxes for each translation.

The lock should unlock itself automatically if, say no input activity was detected in 5 minutes or so.