Add the 'SearXNG JavaScript Style Guide' as one quality gate in the build chain
of the simple theme::
make themes.simple
make themes.simple.test
In the github CI the `themes.all` target enforce a `themes.simple`.
BTW: Remove 'jshint' left overs from 0ee316f3d
Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>