Various packaging improvements, and fix a failing test:
    
    - Switch enable_tests global to a build conditional
    - Add a more detailed description from upstream
    - Assorted spec file tidying, and style adjustments to suit personal preference
    - Convert absolute symlink to relative for executable
    - Use the GitHub tarball as a second source, rather than creating
      tests/docs/examples tarballs from the git tag with a dl-tests.sh script
    - Fix stray executable bits on JavaScript sources
    - Drop patch removing underscores in entity names; this test passes now
    - Fix bundled prod dependencies not actually installed
    - Do not install docs/ and examples/ under node_modules, only under _docdir
    - Add virtual Provides for svgo
    - Hold css-select dependency to '~2.0.0' to work around
      https://github.com/svg/svgo/issues/1315