Server code for configuring JS logging verbosity

I have implemented the server code setting a `meta` header which is used by Javascript to set log verbosity. I have also incremented JS version number in assets (for forcing browser caches flush) and fixed a change by Gattuso which broke the client side code (see upstream issue #5) in `index_main.js`: I'm not sure that it will work as expected, but at least it does not break clients...