API v2: Change boolean value to use true/false

While this is not the OpenProject API, we use true/false for other APIs and for other parameters in the application, like `layout=false`. true/false also seems more intuitive.
API v2: Document some useless code
Merge pull request #1911 from opf/hotfix/api_v2_rewire_parents_value

API v2: Use true/false as rewire parents value
2.5-stable: pdf: fix failure exporting issue pdf with UTF-8 U+FFFD (#17826)

This revision has trunk r13368 and r13369 test.

git-svn-id: http://svn.redmine.org/redmine/branches/2.5-stable@13370 e93f8b46-1217-0410-a6f0-8f06a7374b81