update default partitions size
_Originally created by Tails on [#5834 (Redmine)](https://public-redmine-archive.tails.boum.org/code/issues/5834)_ Given Tails + Wheezy’s ISO size and the actual, real-world incremental update process space requirements (tails/tails#5764), the good old space requirements (2GB medium) and system partition size (1.5GB) may need an update. Feature Branch: feature/bigger-system-partition Parent Task: tails/tails#5490 ### Related issues - **Related to** tails/tails#6015
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