Convert existing Tails devices to UEFI on full upgrade

Originally created by @intrigeri on #6577 (Redmine)

When upgrading an existing Tails device with Tails Installer, turn the system partition into an ESP, just like we already do (at least in liveusb-creator 3.11.6+tails1-3 and later) on initial installation.

Relevant code lines that one can get inspiration from:

partition_type = 'C12A7328-F81F-11D2-BA4B-00A0C93EC93B' 
self._set_partition_flags(1, SYSTEM_PARTITION_FLAGS)

Feature Branch: installer:feature/6577-convert-to-UEFI

Parent Task: #5739 (closed)

Edited May 15, 2020 by intrigeri

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