Enable JavaScript in QtHelpDocumentation for QCH files from doxygen

Summary:
Doxygen in newer versions injects hard-coded JavaScript in the QCH files,
for features like dynamic sections. For consistency with Qt Assistant
allowing JavaScript for now.

Fixing doxygen is only at bug-reported state currently:
https://bugzilla.gnome.org/show_bug.cgi?id=773715

Reviewers: KDevelop, ochurlaud

Reviewed By: kfunk, antonanikin, KDevelop

Differential Revision: https://phabricator.kde.org/D3261