Release 1.7.0+21

Features:
* Track item purchases/acquisitions
* New Stock Movements page combining usages and purchases into a single timeline, with per-group totals and an Initial - Used + Bought = Remaining summary
* Add a user-editable Usage Date Time to usages
* List-row thumbnails with tap-to-zoom for usages, purchases, and maintenances (also inherited by Stock Movements)

Fixes:
* Stock Movements rows were missing their images
* Item list "more actions" popup dividers were invisible on high-density screens
* Purchases showed the creation time instead of the editable purchase date
