Bump livingstyleguide to 1.2.0 release

Upstream fix for sections being rendered mutliple times:
https://github.com/hagenburger/livingstyleguide/blob/master/CHANGELOG.md#120pre2
https://github.com/hagenburger/livingstyleguide/commit/d1838d0a544d0e0cab6c8b68896df7d00a47b014

https://www.openproject.org/work_packages/11314