Cleaned up old auxiliary files, updated references and fixed layout in docs

• .reviewboardrc is no longer needed as we use Phabricator
• porting_todo.txt is no longer needed as porting to KF5 is complete
• createdist.sh is obsolete as we use create_tarball_kf5.rb
• Renamed SVNNEWS to NEWS as it seems to be a useful extended summary of the new features
• Updated broken links
• Updated references and fixed layout in docs
• Small change of wording and layout in TODO, NEWS, ChangeLog files

Differential Revision: https://phabricator.kde.org/D12486