Add CC BY-SA 4.0 International and set it as default

Summary:
New default license (for new documents or relicensed).
Discussed in the previous months on the kde-community@ list
and during Akademy 2017.

Test Plan:
kdoctools compiles. The new license is rendered (with an inline
notice, differently from the "Legal Notice" of FDL).

Reviewers: jriddell, lueck, yurchor

Reviewed By: jriddell, lueck

Subscribers: jriddell, lueck, #frameworks, #documentation

Tags: #frameworks, #documentation

Differential Revision: https://phabricator.kde.org/D6972