[FIX] im_livechat: matching rule in cache

The problem was that the matching rule was evaluated before sending the html
page, and the script was inserted in the page according to the matching rule
(it wasn't inserted if the rule was 'hide_button'). However, the matching rule
wasn't re-evaluated if the page was already in the cache. The page content thus
might depend on a matching rule computed for someone else in that case.

Rather than inserting the script according to the matching rule, we insert it
only if there is an operator available. We then ask for the matching rule in
the livechat widget, by RPC, and we reject its willStart deferred if the rule
says so. We also re-check for operator availability in the same RPC.