URL parameter to set a theme

(just for that session)
https://trello.com/c/B352gd3K/11-theme-url-parameter
various changes aimed at improving performances client and server side and at reducing flickering. Client code auto-update.

- when ws messaging changes, the browser automatically refreshes so that it fetches the new JS
- on vote, only vote information are sent, not the messages
- notable messages list is computed server side on vote and cached
- notable messages list is computed only server side and sent (ids only) when changed
- browser doesn't rerender the messages on vote, only the vote related parts
- when a packet of messages are received, loaders and separations are computed only at end
- various small optimizations