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
issue